I recently threw this together:
http://www.supercoldmilk.com/ac3dplugtut/
I hadn't officially pushed it out there yet (it needs more work), but this should provide an intro to writing your own plugins. You'll need to contact Inivis for the SDK, which actually has some fairly decent examples as well.
Post back here with any issues you have working through it. Because I use AC3D for Windows, it can tend to be a bit Windows-centric (especially the section regarding setting your IDE, which is not only Windows-centric but MSVC-centric as well). If anyone has any notes for non-MS or Mac/Linux IDE's, I'd be glad to add them.
Dennis