THX Does Not Show Up as a Playback Option

I downloaded THX Spatial, registered it, and attempted to follow the given instructions, but hit a snag at the point where you select it as the playback option; as no such driver appeared to exist on my system. I tried reinstalling the programme, however, this did not resolve the issue. I did not have the same issue with Surround 7.1, however, so I do not know why this is happening.