If you have a sound card installed that has a digital S/PDIF port (for example the HT Omega Striker 7.1), then you simply need to assign a source to the input via config|acacia.
1. Open config|acacia by going to config.plethra.com
2. Click on the musical note icon to show the sources for all back-end servers
3. Find the digital input on the sound card (it may be labeled something like S/PDIF digital input)
4. Press the green + button to create a line-in source for that input and give it a name
You should then be able to see the source from the front-end-controller via the "Inputs" source.