Step #1: Determine WHICH SPDIF input you're using. The DM calls these DIG1 and DIG2. Let's say you're using DIG1.
Step#2. Hit ROUTING/INPUT. On the right side of the screen are a myriad of choices. Cursor over to DIGI IN and hit enter.
Step #3: Choose which fader module(s) you intend to assign signal from that DIGI-in port (SPDIF). Let's say it's Module #1 #2 for a Stereo Input. (Be sure you've already paired the intended modules for stereo.) SELect Module #1, and on the left part of the screen highlighted by that selection (Input #1), scroll through the choices 'till you see DIG1/L . Hit ENTER. Next, go down to Input #2, scroll through the choices 'till you see DIG1/R. Hit enter.
Step#4. Ensure Inputs #1/#2 are Assigned to the DM's Stereo Bus for monitoring. A simple push of the button on the upper left panel of the console: STEREO.
Since you haven't stated which DAW you're using, I'll stop here.
CaptDan