Hello and welcome.
-Which model Mac?
-Do you want to use it as an output device or to record...I assume the latter?
Since Macs do not come with an audio input any longer, you will need some form of USB audio input device. The
Behringer U-CONTROL UCA222 would work for you if you connect the record out from the mixer to the inputs. Then, you would connect the USB cable from the UCA222 to the iMac. This would only give you a stereo left/right mixdown connection from the mixer to the iMac.
If you want to have more flexibility in GarageBand, Logic Pro X, etc., then you would need a different mixer altogether. It really depends on what you are trying to accomplish.
C