open a new Logic song and just drag the mp3 onto a track in the arrange window.
If it won't play immediately when you hit the play button, you may have to close the song and re-open it. That's a bug that was supposedly fixed in 7.1.1, but the ****er never disappeared for me and has been driving me out of my mind ever since I paid for that complete piece of **** Logic Pro 7.0.
argh.