The Aux Bus outputs on the AI / RM mixers can not be accessed from DAW software.
You must either use UC Surface to create Headphone mixes with the input channels, then route those inputs to the Aux Buses you want to use (Pre-DAW monitoring).
OR
Create Output channels in software, assign those outputs channels using the FireWire Digital Returns, then route those digital return channels in UC Surface to the Aux buses you want to use for headphone monitoring. (Post-DAW monitoring) This method may introduce latency depending on your Block / Buffer Size settings, channel processing, and the computer's Real Time Processing performance.
You will then need a headphone amplifier to accept the signals. Cabling types will depend on what device you use for Headphone monitoring.
Refer to the StudioLive AI Software Reference Manual, section 2.5 covers Digital Send / Return operations:
Manual Links:
https://pae-web.presonusmusic.com/downloads/products/pdf/StudioLiveAISoftware_ReferenceManual-EN-V6_05042017.pdf
https://pae-web.presonusmusic.com/downloads/products/pdf/StudioLiveAI_Libreria_de_software_Manual_de_referencia_090320165.pdf
