First you need to run an audio cable from the output of your mixer (usually 2 x RCA connections) to the input on the side of your MacBook Pro (1/8" jack). Then, on the window that opens just before you record your mix, make sure that you select the "Built-in Input" option for the Device. You can then play some audio through your mixer to make sure that the levels are registering, and not clipping, on the Input Level meter.
