LAME headache alternative
I read a complicated thread on installing the LAME library for Mac OS. I did get it to work, but it was a headache and Apple warned me against installing it because it might not come from a secure source. (It's probably okay, or someone else on the forum would have warned us.)
There is a simple alternative. Save your score in .WAV format, open it with iTunes, and let iTunes convert it to Mp3. (Control click on the name of the song in iTunes.)