what version of Boxcryptor for Mac are you using?
What is the full name of the Boxcryptor Folder you are trying to access (including the extension)?
Please use this info to make Finder show hidden files: http://www.mikesel.info/show-hidden-files-mac-os-x-10-7-lion/
Now right click on your Boxcryptor folder, select Show Package Contents and see if you can find one of the follwing files:
.encfs6.xml or Keyfile.bcx.