When using the Media Library as a source of images, a Juicebox gallery created with WP-Juicebox will display all images attached to the page or post containing the gallery shortcode.
Once the shortcode (e.g. [juicebox gallery_id="5"]) has been added to the page or post, click the 'Add Media' button (to the left of the 'Add Juicebox Gallery' button above the editor toolbar as seen in Screenshot #1 attached).
Now, just drag and drop your images into the media window which pops up.
Wait for the images to finish uploading and then close the media window via the cross at the top right (do not click the 'Insert into post' button at the bottom right).
That's all you need to do. The gallery is built dynamically at the time it is displayed so there is no need to do anything after uploading the images (other than publish your page or post).
If you want to see the images that are attached to the page or post, then in the media window, select 'Add Media (side menu) -> Media Library (tab) -> Uploaded to this post (drop-down menu)' (as seen in Screenshot #2 attached) and you'll see thumbnails of each image (which you can drag and drop into a custom order if you like). (After re-ordering images, again, just close the media window via the cross at the top right.
I hope this helps.
However, if you continue to experience difficulties, please post the URL to you gallery's web page so that I can see the gallery for myself and hopefully help further.