Assigning outputs

I'm not familiar with this particular device but I would check to see whether you should set something up in the Presonus Control Panel software to achieve this. Otherwise, I presume that as a quick fix simply Soloing the track(s) you want wouldn't give you what you want?

If I get you correctly, in the routing matrix you need to set the track to the appropriate output (mark x in line with the track you only want to hear) and remove any x in the master outputs.

How do I do that on a mac?

The routing matrix is under 'view' in Reaper. I should mention it is not an X as i stated previously, it just fills the box. or as has been mentioned above, if you only want to play back one track, just solo it. there is an M for mute, and S for solo on each track