There is some info on this - go to help>extending dreamweaver in dreamweaver menu. However, this is more of a reference than a useful tutorial. I recently found a book on amazon - Beyond Dreamweaver by Joseph Lowery, which has a small chapter on C Level Extensibility, but apart from this, I can't find any resources or discussion groups related to this subject.
If you're proficient at C programming, then you might not need any more resources - and if so then let me know cos i could do with some help!!