I've never done MP3, but my guess is you export to wav first, then use the Lame. I doubt MC can use Lame in any way, but I might be miserably wrong, so I apologize right away :o/
Anyways, you do want to have one copy of the song in good quality as well, in addition to an MP3 version?
SONAR PE 8.5.3, Asus P5B, 2,4 Ghz Dual Core, 4 Gb RAM, GF 7300, EMU 1820, Bluetube Pre - Kontakt4, Ozone, Addictive Drums, PSP Mixpack2, Melda Creative Pack, Melodyne Plugin etc.
The benefit of being a middle aged amateur is the low number of years of frustration ahead of you.