Instructions to open an existing gallery in JuiceboxBuilder (to edit it):

(1) Click the 'Open Gallery...' button on the 'Start' tab (or select 'Gallery -> Open...' from the drop-down menu at the top of the application).

(2) In the pop-up 'Select Gallery Folder...' dialog window:
Mac - Navigate towards and then double-click the gallery folder.
Windows - Navigate towards and then select (with a single left-click) the gallery folder (not a file within the gallery folder) and click the 'Select Folder' button.

In the 'Select Gallery Folder...' dialog window, it is important that you select the gallery folder itself (and not a file within the gallery folder).
The gallery folder must contain the gallery's configuration file which must be named 'config.xml' (this is what JuiceboxBuilder-Pro looks for).
If you have changed the structure of your gallery since saving it (and use a configUrl or baseUrl to rename or move the configuration file), then JuiceboxBuilder-Pro will not be able to open the gallery.

NB
In the 'Select Gallery Folder...' dialog window, you'll just see folders (that you can navigate within to find the gallery folder, if necessary) but no files.