You don't mention what version of cubase your using, but mute the tracks you don't want exported and see if that works. If it doesn't then copy the tracks you want exported into a new project and export it from there.
I think in the export box there is an option for what type of file you want it exported as...see if mp3 is listed. I would think it would be, don't know for sure because I havn't done it, but I would think it should be.