WP-Juicebox comes bundled with Juicebox-Lite.
You'll need to upgrade the plugin to Pro following the instructions here: https://www.juicebox.net/support/wp-juicebox/#pro

Please note that upgrading to Pro will mean that your galleries are Pro (not branding and no image limit) but the plugin's interface will still only have Lite options. You'll need to enter Pro options as noted in the link above.

After editing a gallery, you usually just need to re-upload your gallery's 'config.xml' file (and any new images in the 'images' and 'thumbs' folders). You might need to clear your browser's cache to see the changes afterwards.

If this does not help, then please try to explain your problem in greater detail and I'll do my best to help you out.

Sorry for any confusion. I should perhaps have written "Please note that upgrading WP-Juicebox from Juicebox-Lite (which the plugin comes bundles with) to Juicebox-Pro...".

The link in my last post assumes that you are already a Juicebox-Pro user (with access to the Juicebox-Pro files) and want to upgrade WP-Juicebox from Juicebox-Lite to Juicebox-Pro.
Please follow the instructions in the link (https://www.juicebox.net/support/wp-juicebox/#pro) to upgrade WP-Juicebox with your Juicebox-Pro files. (Just skip Step #1 as you already own a Juicebox-Pro license.)

(Just for clarification, there is no ready-made Pro version of WP-Juicebox. You just need to download the regular (and only) WP-Juicebox zip package and upgrade it to Pro manually following the instructions in the link.)

I hope this helps.

WP-Juicebox does not allow you to import a gallery from your desktop.
You'd either need to:
1. Use WP-Juicebox to create the gallery within the WordPress interface
... or:
2. Manually embed a gallery (from your computer) following the baseUrl instructions here: https://www.juicebox.net/support/embedd … ery-folder

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That said, you should not be receiving an error 403 when clicking the 'Add Juicebox Gallery' button.
Here are a few tips which should hopefully help.

(1) As the error message you encountered is a permissions error, the first thing I'd do is check the permissions on the /wp-content/plugins/wp-juicebox/ folder (and all the subfolders and files inside it).
I'd also check the permissions on the /wp-content/uploads/juicebox/ folder (where the gallery configuration files are stored).
Default permissions of 755 for folders and 644 for files should be fine.
You should be able to check and change permissions using an FTP program (such as Filezilla) or via your hosting account's online file manager.

(2) Maybe you have a security plugin which is somehow interfering with WP-Juicebox's functionality.
If you do have any security plugins installed, then please try temporarily disabling them (at your own risk) to see if this allows the 'Add Juicebox Gallery' button to function as it should.
If you find a security plugin which does prevent WP-Juicebox from functioning correctly, perhaps the plugin has the ability for you to whitelist WP-Juicebox to bypass its security measures.
I'd also extend this troubleshooting tip to all plugins other than WP-Juicebox (not just security-related plugins) just in case you find one that is conflicting with WP-Juicebox.

(3) If you use the BulletProof Security plugin, then this may be the cause of your problem and you will need to add a skip/bypass rule to the .htaccess file in your root directory as follows:

# Juicebox skip/bypass rule
RewriteCond %{REQUEST_URI} ^/wp-content/plugins/wp-juicebox/ [NC]
RewriteRule . - [S=13]

(4) Please check all .htaccess files on your web server (there may be one in your root directory and also perhaps in your WordPress directory) for any entries which might be causing a problem, such as:

<Files *.php>
    deny from all
</Files>

If you find such an entry, either comment it out or remove it.

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If none of the above suggestions help, then all is not lost.
You can still create a gallery on the 'WP-Juicebox -> Manage Galleries' page.
After creating a gallery here, copy the shortcode presented to you and paste it into a Gutenberg 'Shortcode' block in your page where you'd like the gallery to appear. You'll find the 'Shortcode' block in the 'Widgets' section.

Once the gallery shortcode has been added to the page, you'll need to add images to your gallery.
A WP-Juicebox gallery displays all images attached to the page containing the gallery shortcode. You can attach images to a page as follows:

1. Click the 'Add Block' button (the '+' icon) to add a new Gutenberg block to your page.
2. Select a 'Gallery' block. (You'll find it in the 'Media' section.)
3. Click the 'Media Library' button within the 'Gallery' block.
4. Drag and drop the images that you want to attach to your page (i.e. the images that you would like to be displayed in your WP-Juicebox gallery) from your hard drive into the media window.
5. Wait until the images have finished uploading.
6. Reorder the images (if necessary) by going to the 'Create Gallery -> Media Library' section (within the media window), selecting 'Uploaded to this post' from the drop-down menu and then dragging and dropping the thumbnails into the order you require.
7. Close the media window via the cross at the top right. (Do not click the 'Create a new gallery' as this will create a native WordPress gallery in addition to your WP-Juicebox gallery.)
8. You can now safely remove the 'Gallery' block from the page. (Using a 'Gallery' block is just a convenient method of attaching images to a page.)

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If you want to upload a gallery from your computer and embed it into your WordPress site, then you'll need to do so manually (bypassing WP-Juicebox).
I would recommend using the baseUrl method of embedding documented here: https://www.juicebox.net/support/embedd … ery-folder
Essentially, once you have created a Juicebox gallery on your computer with JuiceboxBuilder-Pro, you would upload the complete gallery folder to your web server and paste the baseUrl embedding code into the body of your WordPress page or post. It does not matter where on your web server you upload your gallery folder to as long as the two paths in the embedding code (the path to the 'juicebox.js' file and the baseUrl itself, pointing towards the gallery folder) are correct.

Here's an example (which assumes you use the current version of WordPress with the built-in Gutenberg Editor).

(1) Create your gallery with JuiceboxBuilder-Pro and save it to a new empty folder named "my_gallery_folder".

(2) Upload the entire gallery folder (not just the contents) to the root directory of your web server (using an FTP program such as Filezilla).

(3) Create a new Custom HTML Gutenberg block in your WordPress page or post (in the 'Formatting' section) and paste the following embedding code (changing the gallery dimensions and background color if you like):

<!--START JUICEBOX EMBED-->
<script src="/my_gallery_folder/jbcore/juicebox.js"></script>
<script>
    new juicebox({
        baseUrl: "/my_gallery_folder/",
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "600",
        backgroundColor: "#222222"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

The leading slashes in the paths above denote your root directory so the code above will work without modification as long as your gallery folder is named "my_gallery_folder" and has been uploaded to your root directory.
You can replicate this process as many times as you like (for other galleries).
Of course, for subsequent galleries, you'll need to change the name of the gallery folder (so that each gallery folder has a unique name) and change the two instances of the folder name in the embedding code, but that's all.

I hope my notes above help.