How to call the illustrator plugin?

Are you talking about invoking part of the core Illustrator product from the SDK? If so, a great deal of it is available through various headers relating that part of the product (e.g. font related functionality can be fond in AIFont.h).

If you want to more specifically invoke filters or menuitems as found in the runtime, you probably want to use AIActionManager.h to fire off an action. if that's what you want, you can find examples and details on these forums elsewhere (I've posted in numerous threads on the subject).