In simple mode, only one BC Classic folder can be used at the same time. In order to use 2 or more BC Classic folders at the same time, you need to switch to the advanced mode.
Opening an existing BC Classic folder should be as easy as clicking on "Open" in the toolbar of the BC Classic preferences. BC Classic then tries to auto-detect your existing BC Classic folders (they need to have the ".bc" extension for this). If auto-detection does not work, you can always manually choose the location of your BC Classic folder:
1) Click on "Open" in the toolbar
2) Choose "Custom" for "Location or Folder"
3) Select your BC Classic folder and click "Open"
If you want, you can also restore the Boxcryptor preferences of a previous point in time from a preferences plist file:
a) Quit BC Classic if it is running
b) Open Terminal
c) Execute: "defaults import com.boxcryptor.mac.BoxCryptor /PATH/TO/com.boxcryptor.mac.BoxCryptor.plist"
d) Start BC Classic again
I hope I could help you. Best regards,