I want to open the gallery I just created (and uploaded successfully, ta-dah)...
If you have uploaded a complete gallery folder to your web server, then you should be able to view the gallery on a web page of its own by opening open the 'index.html' file (from directly inside the gallery folder) in your browser.
For example, if your gallery folder is named "my_gallery" and you have uploaded the complete gallery folder to the root directory of your web space (in this example, on a domain named "example.com"), then the gallery can be viewed by entering this URL in your browser's address bar: http://example.com/my_gallery/index.html
... there's no files in there that can be opened directly by JB.
You can view a Juicebox gallery by opening the gallery's 'index.html' page in any modern browser (Chrome, Edge, Firefox, Internet Explorer, Opera, Safari).
Otherwise, you can embed the gallery into an existing web page (following the embedding instructions here) and open that web page in a browser instead.
If you are trying to open an existing gallery in JuiceboxBuilder-Lite (for example to edit it), then you need to click 'Open Gallery...' (or 'Gallery -> Open...' from the drop-down menu at the top) and then navigate towards and select the gallery folder itself (not a file within the gallery folder) before clicking 'Select Folder'. The gallery folder is the folder which contains the gallery's XML file and the gallery's XML file must be named 'config.xml'. (If you have renamed or moved this file, then the gallery will not be able to be opened by JuiceboxBuilder-Lite.)
I hope this helps to clarify things.
Please let me know how you get on and if I can be of any further assistance.