AC3D 6.5 has now been fixed to also work under Mac OS X 10.4.
If you are running Mac OS X 10.4 or using Mac with a PowerPC processor, you should update to this version. (PowerPC users also benefit from working Greeble, Lightwave and UVMap plugins which were previously Intel only).
If you are running Mac OS X 10.5 on Intel, there's no real need to download AC3D again.
Download it from:
www.inivis.com/downloads.html
Thanks to all those who helped testing and debugging the OS X 10.4 problem.