According to his System Specs, he's using HDMIAre you on HDMI or Spdif? Multichannel PCM would only work with HDMI, with Spdif you'd have to downmix to stereo. But then it is better to just bitstream DTS Core.
and a hint for you: MPAR is able to encode the audio again to AC3 for Multichannel support on SPDIF