While a USB adapter or a USB mic is a good idea, if you're in a pinch and have a DV camera with mic input around, you can get away with it. Connect the mic to the camera, put the camera in "Camera" mode, connect the camera to the Mac, and use FCP's Voice Over to record DV Audio. If you have an HDV camera, flick it into DV mode for this. Don't forget to disable any "auto-off" feature of the camera and eject the tape.
However you record, clean up the audio for at least a light Noise Reduction and probably a Noise Gate in Soundtrack Pro.