Clive...I have PowerTracks version 8...I just took a look and didn't find a way to convert to MP3 from inside the program (It does have a "Merge audio tracks to WMA file" option)...
You may need to use a separate wav -> mp3 converter program (after merging your tracks into a wav file from inside PowerTracks). From time to time people ask about converters on this board and are usually given several free program suggestions.
What I have been doing (when using Powertracks) is choosing the option "Merge audio tracks to stereo wav file" and then opening that wav file in Cakewalk Pyro 2004 (which I downloaded for around $30). I use Pyro as a "poor man" mastering tool, it allows one to EQ, boost volume, and add effects to the stereo mix. Pyro also lets one convert between wav, mp3, and wma, this is what I use to convert my "mastered" WAV to MP3.