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If the mixer has audio line-out, and your MacBookPro has an audio line-in you can record the output of the mixer in Garageband. Your mixer has two outputs - 1) 2x RCA and 2) 2x 1/4' TRS. You will need a cable with one pair of connectors matching the mixer output you want to use, and on the other end a single 3/8' (aka 2.5mm) male stereo plug to connect to your MacBookPro.
In GarageBand you will record from the audio input. There is no 'connection' between the mixer and GarageBand per se.
Note: You didn't say what model MacBookPro you have. In the current lineup, only the 15' MacBookPro still has audio line-in. Check yours to make sure it has an audio-in port.