AMD decided to leave it's x64 users out in the rain on this one...
Q: Why does ATI Video Converter not work on 64-bit operating systems when so many PCs are moving in that direction?
A: While the move to 64-bit is well underway, 32-bit OS’s satisfy the mainstream consumer and the ATI Video Converter was to fill that segment first. We are evaluating our roadmap for future improvements, including the possibility of offering a 64-bit solution.
http://hothardware.com/News/AMD-Responds-ATI-AVIVO-Video-Converter-Feedback/