AU Inst doesn't appear as input option in aux track
I can't figure out why my instance of Omnisphere won't appear as an input option to an aux track (I've done this before and it worked fine). I created a multi-timbral instance of Omnisphere (appeared as inst1 with 8 tracks) then I added 8 aux tracks (so I could do surround mixing.) when I selected input for the aux track there were input options for Ableton Live and Sibelius, but nothing for my Omnisphere instrument (Inst1.) I've obviously forgotten to do something, but I can't figure out what it is. Please help.
Mac Pro, Mac OS X (10.7.4)