JuiceboxBuilder-Pro can open local galleries only (i.e. those on your hard drive).
The application is not able to open galleries which reside on a web server.
Can you help me understand the workflow please?
The following workflow would work:
(1) Create a Juicebox gallery using the Lightroom plugin by clicking the 'Export' button to save the gallery to your hard drive.
(2) Open the gallery in JuiceboxBuilder-Pro and edit as necessary, re-saving the gallery on the 'Publish' tab when finished.
(3) Upload the gallery folder to your web server via FTP.
If you do wish to edit a gallery which has been uploaded to a web server, you could use svManager.
This is a PHP web application which allows you to create, import and edit galleries online via a web browser interface.
All Juicebox-Pro configuration options are customizable via svManager's interface.
If you are interested, you can try out the online demo version of svManager here.