To use the S/PDIF input, your interface will need to be clocked via S/PDIF to the device you're getting input off of. Unplug the S/PDIF input on your interface and open your interface's control panel. Now set your Clock Source to S/PDIF. When the sync light begins flashing, plug your device's S/PDIF output into your interface's S/PDIF input. When the light stops flashing, you should be ready to record the S/PDIF input.