Hey Andy,

Aftertouch is not anything special, its sends a MIDI message just like turning a MIDI knob does. What you need to do is tell the synth to listen to the aftertouch midi message, and use it to control <your-parameter-here>.

If you cant map aftertouch to what you want, try converting it to a MIDI Control Change (MIDI knob turn) type message, which has become almost a standard in controlling software parameters.

You didn't mention what it is that you want to do with the aftertouch?
Cheers, -Harry