OK, I figured it out.
The documentation says that 3rd party plug-ins will work for Photoshop 7 in OS9, but not in OSX (it's up to the third-party plug-in companies to update their plug-ins so that they'll work in OSX).
Just put the KPT or whatever plug-in folders inside the Photoshop 7 "Plug-ins" folders, and you're good to go (for OS9 anyways).
Cheers,
- Corey