I'm not very familiar with Adobe Audition but it should support VST plugins. If it does you can just insert Absynth as a plugin and record it with your midi keyboard. For Reason you need to use the Re-wire function. I haven't done this in a while but I used to do it in Garageband. I think you'd open Audition, then Reason and Reason should be basically hooked up in slave mode. This means Audition has control over track postioning, playback and recording. I'm not entirely sure how Audition handles this, but with Garageband you would record your stuff in Reason and when you did a mixdown or bounce to disk it would combine the Reason stuff into the final mix.
