Killthee is dead on. If you change the setting under the "Import" tab to MP3 Encoder, that will do the trick. Highlight a bunch of songs, right-click, and the context menu will have "Convert selection to MP3" as one of the choices.
Protip: Copy your original files somewhere else first. Once you do the conversion, you will lose the originals (if memory serves)...and converting from PCM -> AAC -> MP3 is a guaranteed way to make the music sound like ass. After you get done with the MP3 version, you'll want to restore the AAC for best sound quality.