Ableton does not, and probably never will, support MP3's.
This has been requested many times on the Ableton forums and it always gets a negative. You'll have to convert to WAV if you want to use it. Cool Edit Pro 2 can batch convert WAV's, as many as you like in one go
fair play, thanks for the heads up
I always use .wavs anyway.. but thought you could do mp3's as well..
anyway, decoding mp3s would be a processor drain, and of inherently worse sonic quality.