I also use audacity, and I also can't export to mp3, I think there might have been a charge in order to use that feature. what I do is export to .wav then convert it to mp3 using a free downloadable converter. I cannot remember what the name of mine was, nor can I find it on my computer, if I find it I'll let you know what it was called.

in the meantime you can download a free trial of dBpoweramp which will convert the file for you