the way you describe it, you want to connect a microphone to your ada8000 and get the signal into your computer, right?

if you want to set the rme to master (which is a good idea), make sure it's using a samplerate that the behringer can do (i.e. 44k1 or 48k).
if the wc in doesn't work (which wouldn't surprise me), try running two adat cables between the boxes (in and out) - that way, the behringer can sync to the adat out of the multiface.

you do get sound in and out of your multiface via the analog i/o, do you? because that's the first thing you should troubleshoot, before adding another potential source of trouble...