Synths such as FM7 (based on the Yamaha DX-7) and Absynth when installed will show up under the Generator section when you go to choose an instrument (click the small arrow at the bottom to show more details). After choosing a Generator, click on the Edit icon, and you get the synth available to choose sounds or edit. Once you have what you want, you can save the instrument as you would in the GarageBand area to avoid going back through those steps for a sound.

