Hmm.. I can't remember how I did it, but it seemed obvious at the time - although having said that, not everything from apple always is, with odd sequences as you describe often happening.
I have a network based on an Airport Extreme, and it is that network that my TC is added to. I don't know if it matters that my original network is apple-based rather than being based on another router.
Under the 'network' tab, I have the router mode option set to 'Off(Bridge mode)' if that helps?