If you're recording all the instruemtns all at once, then you can't mixdown, as you've already mixed them down to stereo using your mixer. If you're tracking each instrument separately, then that's not a problem. Could you clarify exactly what you're doing?
I haven't used n-tracks so I don't know the exact steps to mixdown but as long as you don't need to run ther mix through any external hardware like an eq or compressor you should be able to just select a mixdown function in the software and it will create a finished .wav or .mp3 file.