Ahh, ok. Thanks for clarifying. I'm not very familiar with using Focusrite as an interface (I have one, but not as an interface, I'm using it as a standalone analog <-> ADAT converter). I think to get the SPDIF input into your DAW you don't need to set anything up special, it should already be routed to the appropriate channels (from what I can tell from their docs). However, if you want to direct monitor via hardware (I.e. without going via your DAW) then GlennO's advice is true.
Whether you want to hardware monitor is personal preference. Personally, I monitor everything via my DAW because the latency is low enough and it simplifies my setup. I can record, monitor and playback in any combination all via DAW control without changing hardware device settings - and I much prefer that workflow. But, again, that's down to personal preference.