What connection do your PC speakers use? 3.5mm, I'd assume...
You'd need an adapter/cable that goes from 2 x 1/4" TS male (or TRS) to 3.5mm TRS female. If the FireFace UC is like the UCX, then the main speaker outputs default to analog outs 1 & 2. So plug the 2 1/4" plugs into analog outs 1 & 2 and plug your PC speakers into the 3.5mm jack of the adapter.