If you're smart, you don't. You *can* hook up a MIDI keyboard to Au3, and you can record stuff, but the problems you will inevitably encounter will lead you to conclude, like everyone else that has tried MIDI in Audition, that it's better left alone. I record MIDI in Reaper, which is good and cheap. I then convert the midi file to a wav file and import it into Audition, which is a great program for everything except MIDI.