Hi, and welcome to the Forum.

The mixer and the computer must be connected via USB or FireWire, then UC Control/Surface will show the mixer model, which Capture then will "see" as audio interface. Without a mixer as audio interface, Capture will "see" the computer as audio interface, but it probably will not be able to use it, maybe just for two track recording.
Make sure that the mixer is available as audio interface, then Capture will configure itself according to the settings of the mixer you are using. Capture is able to record 64+2 tracks - with the appropriate mixer (SL 64S)...