Hi--
I have the modem router address and the MAC number
from the modem.
If your state is like mine, you just need the MAC address of the cable modem. If I were you, I'd just give up on the installer and call Comcast.
Here's what's happening: when you hook up a new cable modem to Comcast's network, you get stuck in what's called a "walled garden" where, as you've found out, it tries to get you to download and install some software. My experience is that the software stomps all over your network settings, creates general havoc (including setting IE as your default browser), then, if it works at all, eventually dead-ends you at a last page where you're told to call a 1-800 phone number to call, with your cable modem's MAC address. You'll talk to a real live person for about 30 seconds, then you should be good to go. I think the number is 1-888-COMCAST.
charlie