At this point, the wizard will go hunting for the modem. In the previous configuration, it would have found the modem as a router, and then confusion would have ensued. Now it finds it as a modem, and all is well.
In the following steps, it will ask you to unplug the modem. This will be departing from the steps in the User's Guide, but follow along. The installer will "talk" to the modem for some time during this process. Just be patient. When you see the activity light flash on the modem, it's about half-way done.
Eventually, it will ask you for your network username and password. Enter them, and then watch as it does some more configuring. It will also ask you to create a login password for the router and a network name and security key for the wireless network. It will also render some advice about WEP and WPA security.
When this is done, the router will be responsible for logging your system onto the network as well as distributing traffic among up to four wired devices and an untold horde of wireless devices.