Easy, download your choice of MIDI files, then just drag the MIDI file onto the GB editing window. It works fine for me because I have JamPack 4, so either it will choke or all your tracks will be "Flute" and you will have to either guess what the instruments are suppose to be, or use Quicktime Pro 6 to ID them (Rondo would help here as well), or, if you are lucky, the author has already ID'd the track with the instrument name. You might want to start with just a piano piece like something from Beethoven or Chopin.
This piece was created with JamPack 4 using this method. You can hear the inferior instrument sampling, however, JamPack 4 helped improve the importing process by assigning the tracks properly as well as providing orchestral icons.