...not being a Mac-user I cannot vouch for the accuracy of this, but one assumes all menus are similar. When selecting "Save File As" from the "File" menu, either use the drop-down "File Type" menu or manually add the ".MP3" suffix to the filename. This will automatically invoke the codec necessary to process into mp3. It should give you all the usual choices about VBR or set bit-rates or what-have you.
Just in case you don't know, there is a significant loss of data in the compression from .WAV to .MP3, so if you know you'll be editing the file again either leave it as .WAV or use the highest bit-rate settings.