If you are talking about the SL AI mixers, please read section 4.10 "Monitor Bus" in the StudioLive AI Series Owners Manual. You find the manual on the download page of your product. The SL boards have buttons, which also can be handled by UC Surface, but pressing buttons might be easier to understand. You also have to decide WHERE/HOW you want to hear. Mostly you will want to "solo" a channel in your headphones, which need to be connected to your board and the volume level has to be raised. It's all explained in manuals section 4.10.