No luck. Still not working.
AIR is version 4.0.0.1390
JuiceboxBuilder-pro is 1.3.3
I tried uninstalling JuiceboxBuilder-pro via the installer, deleting /Users/your_username/Library/Application Support/Adobe/AIR/ELS/JuiceboxBuidler-Pro/ then reinstalling JuiceboxBuilder-pro and I am still seeing the same problem. The app starts and displays the "Start" screen but then won't load or create a gallery. It doesn't crash or anything, it is still there, and clicking "Open Gallery" again will again display the file selection dialog, but selecting something from that dialog continues to do nothing.
It had been working fine until last night and I haven't changed or installed or updated anything recently. I had been running Photoshop CS5 when the problem began (I usually use CC but still occasionally need CS5 for its scanner support and this was almost certainly the first time I've run it since buying Juicebox Pro) but can't think of anything otherwise that I haven't done tons of times before.