Make sure that all versions of UC Surface and mixer firmware are on the most current version as shown in your presonus account. The mixer needs to be connected via firewire to use the smaart wizard.
Now in UC Surface, main mix in focus, scroll all the way to the right on the faders, and you will see a fader for talkback. Select that, and in the fat channel area, you will see a drop down to assign one of the analog inputs to talkback. Do not use channel 1 or 2 for talkback, as they are used by the smaart wizard for processing.