Im not real familiar with mac, but if you get some audio editing software (you might even have a wav editor or somethin similar already in your computer) then set it to record to the line input. Then hook the main outs of your mpc into the line input of your computer. Press record, play the beat, then stop recording when the beat is done. Then edit it with whatever you need to do whether its compression or eqing or just adjusting the start and end points. Then burn.