Within OctaMED, you need to make sure of several things
1) that midi is enabled in the midi menu (v6+)
2) that you have an instrument set up to play on a MIDI channel. Channel 10 is a good start if your kit is GM capable, you can usually hear percussion a mile off
3) that the instrument volume is not zero
4) the instrument may require increasing the transposition, the midi note numbers tend to start at a much lower octave
Then try tickling your amiga keyboard (as you would to play a sampled instrument) and see if your MIDI device makes a sound
Hope this helps...