This is a little tricky at first, but it's not too hard. You didn't mention what version of Cakewalk you're using, though. Here is how you can send only the Reverb Aux buss to a new track in Cakewalk 9:
* Set your reverb aux buss to Pre fader
* Pull all the track faders feeding the aux buss to 0
* Mute all tracks not feeding the aux buss
* Goto "Tools | Mixdown Audio | Bounce to Tracks"
* Select a free destination track and whether you want stereo or mono
* Uncheck everything except "Aux Returns"
* Click, and brew a pot of coffee
Good luck