Not sure how you are trying to export the tracks, but the easiest way I found to export only certain tracks into a wave file is to first disable the tracks you don't want in the mix. (Right click on the left side of the track, the part with the record arm on it and choose "disable track")Then make sure the tracks you want to export are not muted, then click song > export mixdown > choose your settings > ok.
Also if you are running artist or prime, you cannot export your recordings into MP3. If you have artist you can buy the MP3 extension that will let you do that.
