Well, I dunno about this one. Sounds like OS X isn't supported.. yet. Write to Macromedia and complain.
If you have the OS 9 plugin file, you may try to drop it in the IE 5.1 Package's internal plugin directory, like someone else did with the Flash player plugin, I believe. It may work, I guess it's worth a shot. Check out the newer posts and see if you can find the thread I was talking about. Good luck!