Try it again. You make a new GB project, drag an MP3 file in, and it imports.
Is the file really an MP3 or other format supported by QuickTime?? Purchased iTunes tracks in particular are not allowed.
Another thing that can screw up imports is if you have an absurdly low or high bitrate on your MP3. TRy converting it to something more normal in iTunes or whatever if that is the case. AIFF would be a good conversion choice, that is what GB uses natively.