Did you amend the aircraft.cfg file? As Ian and BB686 pointed out, a lot of CFS2 (and, indeed, FS2002) models will work in FS9 as long as they were made using FSDS2 or Gmax; but the CFS2 aircraft.cfg doesn't include some of the entries that FS9 needs, specifically the UI Variation, UI Type etc. So, if you didn't add those it will either not appear at all or, sometimes, appear as an "Unspecified Type" at the bottom of the aircraft select menu.
Porting over CFS2 aircraft can be a little time consuming, as you need to make those additions to the config file, but certainly can be worthwhile; for example, the Burnage and BrunoSK Mosquito's work nicely, with a minimum of tweaking and there are plenty of repaints out for both. Similarly the Rebuffat Spitfires work well, which is lucky for those of us who want to include a Mk XII on our flightline.