On PC's it appears as a USB audio device (sort of like a giant sound card) when you connect it. According to the 990's manual, the USB port is limited to PCM stereo only. I'd be curious to know if OSX also recognizes it as an audio device (it would seem likely), but you'd still have the PCM stereo limitation.
For a Mac Mini setup, you might look at the
M-Audio Transit . It connects to the USB output of a PC or Mac and provides an optical connection that will pass Dolby Digital and DTS, and it costs about
$80 .