If you encountered this error, you should simply obtain the DRM keys from your content provider where you purchased audio/video files.
To do this, simply log into your account with that service and then attempt to play your audio/video files in the Windows Media Player. It should automatically prompt you to download the licenses to play the music. After that you may try to use FairUse4WM again.
FairUse4WM does not work for me (Error: No keys loaded). Can anyone help?