Those speakers don't have an integrated surround decoder, which you need. I have heard that the Creative SoundBlaster Extigy USB audio unit will work on Macs, even though it's not supported on the Mac.
Only Logitech's $400 speaker sets (the Z-5500 Digital and the Z-680) have the integrated decoder.
For half that much, you could buy a cheap home theater receiver with a Dolby Digital decoder, and with a few adapter cables, hook your speakers up to its unamplified outputs.
tooki