Try navigating inside the gallery folder before clicking the 'Select Folder' button.

Do you see the following message?

Make sure that your gallery's XML file is named 'config.xml' and is located in the root of your gallery folder.
If you have renamed or moved the file (for example if using a configUrl or baseUrl) JuiceboxBuilder-Pro will not be able to open the gallery.

If none of the above helps, try reinstalling JuiceboxBuilder-Pro.
Make sure that you have the latest version (v1.3.2). Instructions on how to get the latest version can be found on the Upgrading Juicebox page.
After uninstalling the old version and before installing the new version, manually delete the following folder (if it exists):

• Mac: /Users/your_username/Library/Application Support/Adobe/AIR/ELS/JuiceboxBuilder-Pro

• Windows (XP): C:\Documents and Settings\your_username\Application Data\Adobe\AIR\ELS\JuiceboxBuilder-Pro

• Windows (Vista, Win 7, Win 8): C:\Users\your_username\AppData\Roaming\Adobe\AIR\ELS\JuiceboxBuilder-Pro

I have the same problems.
I already deinstalled and reinstalled juicebox builder - it won't open my old gallery file.

Now I tried starting a new gallery and juicebox builder puts all folders, files and shortcuts from the desktop into tha gallery folder. Why?

After saving the galery file juicebox builder still won't open it.

Please note that you should navigate towards the gallery folder (not a file within the folder).
What exactly happens when you try to open your existing gallery?
Does the 'Open Gallery...' button open the 'Select Gallery Folder...' window?
Does JuiceboxBuilder-Pro freeze when trying to open the gallery?
Are any error messages displayed on screen?

JuiceboxBuilder-Pro should save a gallery to the location specified in the 'Gallery Folder' field on the 'Publish...' tab.
You can select the location using the 'Browse...' button.

If, as you say, the 'Open Gallery...' button opens the 'Select Gallery Folder...' window, then you should be able to navigate towards the gallery folder that you want to open.
Is it when you click the 'Select Folder' button that nothing happens?
When you click this button one of two things should happen:
(1) The gallery should open, or...
(2) The ''config.xml' file not found in the selected Gallery Folder. Check the selected folder is a Juicebox gallery folder.' message will be displayed.

Please note that the gallery folder must contain a file named 'config.xml' in the root of the folder. If you have renamed or moved the file (for example if using a configUrl or baseUrl) JuiceboxBuilder-Pro will not be able to open the gallery.

That sounds very strange and should certainly not happen. No other users have reported such a problem so the issue seems to be specific to your own system, setup or workflow.
Where are you saving your gallery folder to? Try a different location. Create a test folder in the root of your C:/ directory and try saving your galleries in there to see if this makes a difference.

If you are not already using the latest version of Adobe AIR (v3.8), I would also try upgrading it.
It can be downloaded from this web page.

If you like, please zip a gallery that you cannot open, upload the zip file somewhere and post a download link so that I can take a look and see if I can find any problem with it.

I have not been able to replicate this behavior but I am glad that everything seems to be working OK at the moment.