I'd like to record a few instruments on the original mp3 track of a song... I just can't find how to match bpm and click once imported the mp3 in logic.
I'm not sure what version of logic you're using (I have logic 8), but have you tried inserting a bpm counter?
for the track you're interested in...
inserts -> metering -> bpm counter -> stereo
then play the track, and it should find the beats per minute after a few seconds...
a video tutorial on youtube :