A little time on google suggests that the M17x does not support DDL over the optical. So don't bother with that adapter.
Sucks. Yes, getting set up with a PC on the Mixamp is a bit of a pain; it's intended for consoles, and consoles put out Dolby Digital out of the box (well, except the Wii). But honestly, I'm placing this on Alienware/Dell. Whoever designed a "gaming" laptop with SPDIF out but without DDL support needs to be beaten with a rubber hose.
Black A40 UP Walrus | White A30 PAX 2010 LE | 2010 Mixamp
Green A40 BXR | 2011 Mixamp | I Should Sell Some Of This Crap