Getting to this thread late, but everything Mark said is correct. It is called "Walled Garden" when a modem is on the system, but has not been provisioned for use. The CMTS (not DeMarc, that's for phones) or Cable Modem Termination System, is what you were thinking of.
When your modem is in Walled Garden, as Mark stated, you can only ever get to the Comcast self-install kit page, where you're supposed to be able to provision the modem yourself. But I remember way back when I first got Comcast (before I worked there) it didn't work and I had to call anyway. Once they put that modem on your account you should be golden.
You weren't trying to use that cheap-ass "gold-plated" Radio Shack crap cable were you? If you need a good piece of RG6 let me know, I've got some extra laying around.