On the 13 inch MacBook, the "headphone input" is actually a dual input/output port. This is from another site (I think referencing the manual): Choose Apple () > System Preferences, and open Sound preferences. Specify whether you want the port to be used for audio input or audio output. So, once you've done this, then you can run a 2xRCA to 1/8" jack from your mixer to your MacBook's audio input port.
