If you want to bounce the track only to save CPU, you can just hit the "Freeze" button (the one that looks like a ice crystal icon) in the track header. If it's not available in your track header, control-click (or right-click) the track header and choose "Configure Track Header..."
If you want to record a software instrument to an audio track do the following :
1-Insert a soft inst on a track and send its ouput to a bus.
2-Select the bus in the input of an audio track.
3-Make sure the audio track is "record enabled" and select the inst track.
4-Hit Record and play the inst (or let play the sequence)... it will be recorded as an audio file.