Lorie,
I assume you're working in a DV sequence; DV requires 48kHz 16-bit audio and MP3 is a compressed format with not nearly enough quality. The easiest thing to do is open your MP3 in iTunes then change your IMPORT settings (in the Prefs) to AIFF encoder and under CUSTOM select 48kHz, 16-bit stereo. Select the MP3 then under ADVANCED you'll find a CONVERT TO AIFF command. Import your new AIFF file into FCP and replace the MP3 on the timeline and your rendering woes are gone.
Also, if your MP3 was taken off a CD then I suggest you start over and rip that track as an AIFF from the CD so you start with the highest quality. The MP3 you have is already compressed, and the upsampling to AIFF will not improve that.