I forget the exact command, but you want to Export Audio or Export to Wave, or something like that.
Mute or archive any tracks you don't want included in the mix. Then highlight all your tracks and find the Export Audio command.
In Sonar that command is in the File Menu. But my recollection is that it was located somewhere else in PA9.
BTW, that's if you want to do an external mixdown. If you want to do an internal mixdown, just highlight the appropriate tracks and use the Bounce to Track command.