It may be due to a sample rate mismatch that you could not get signal from the DVD. Almost all DVDs are at 48khz. So, if you were set to 44.1khz, it may not recognize/accept the input.
Well the MicroTrack can record via S/PDIF up to 96k and it auto-senses the input signal so I would have guessed it would have worked. I theory that just popped into my head is that maybe the player wasn't outputting on S/PDIF expect when playing a CD.
Maybe plug something else (like a receiver) in to that output and verify something is actually coming out when playing a DVD (I'm guessing that's what you were playing before?). You might even unplug the analog cables just so you're 100% certain that if you do hear something that it's coming out of the S/PDIF.
Lastly since the MicroTrack is new to you, understand that you must have the signal going already 'before' pressing record on the MicroTrack so that it has a clock to lock onto and also remember that you cannot monitor the signal on the MIcroTrack's headphone (or RCA jacks for that matter). Only analog recordings can be monitored.
Hope some of this helps and hope you have fun with your new toy.
J.T.