Mix outputs are not accessible from Software. All StudioLive mixers operate in this fashion, as would most Live Sound Digital Mixers.
You must use your Digital Returns to get Audio back to the mixer, then route your audio using UC Surface to the Mix Bus you want. Do note that software monitoring can introduce latency depending on your Block / Buffer size set at the driver. The lower the size, the better the latency, but the higher the CPU processing. Universal Control also has a 'Safe Mode' for Block size that makes it easier to adjust.