Okay, billyworry, maybe i can help you -I thought i did before, but oh well...
The Mackie 12fx mixer is analog only, so you need . 1 pair RCA Cables for recording and other pair for playback. Connect one pair to the tape in and one pair to the tape sockets on the mixer. Main Out is for hooking up your speakers. After plugging in the tape sockets, you may want turn on the : tape to main button. Under system prefences, you should make sure that sound is set to Line out and in around the back, not Internal Speaker. Also go into "Alerts", also under system preferences and make sure it's set to play them on Internal Speker. There should also be a slider knob to adjust the level of the imputs as well..you can connect headphones to the control room output.
Manual can be found here...
https://mackie.com/sites/default/files/PRODUCT%20RESOURCES/MANUALS/Owners_Manual s/Mix5_8_12FX_OM.pdf
You cannot use a standard Mac microphone as the plug is somewhat different, but there is an adapter for it. You can however use a conventional Mic that supports Phantom Power., I'd suspect that the inputs would be on 1 and 2
thats all I have
John B