This should do the trick:
1 - Import the MIDI file.
2 - Select the track you want to apply your instrument to.
3 - Right-click the selected track and open it in the inline MIDI editor.
4 - In the drop-down box on the left under the editor keyboard, select "Bank/Program Select".
5 - At the start of the track, there should be a vertical green bar with numbers next to it like 0-0-n, where n is the MIDI instrument ID. This is the MIDI instrument you want.
6 - Go to
http://en.wikipedia.org/wiki/General_midi to find the instrument associated with the instrument ID.
7 - Rock out.