This is most likely because iMovie doesn't support the compressed .mp3 format for use in editing. You need to use uncompressed audio, like you'd get directly off a CD or your DV camcorder. The format for this is AIFF (Audio Interchange File Format), most likely you'll either want a 44.1 kHz sample rate (this is what CD's are) or 48 kHz sample rate (audio from your DV camera).

You can use iTunes to convert the .mp3 to AIFF (This is basically the exact opposite of "ripping" it). Select the file you want to convert, and choose "Convert selection to AIFF" from the Advanced menu.

Then, in iMovie, select "Import File..." from the file menu, change the drop down selector to AIFF audio, and locate the file you used iTunes to convert.

You're all set.

Hope this helps.....