The most common configuration is to have the WAP act as a DHCP server. This is on the LAN side of the router setup. So your TCP/IP settings on the iMac should be to use the DHCP server. You may or may not need to add DNS information, depending on your DSL provider.

BTW, just curious--why did you buy a wireless router if your iMac is wired?

As for the Oreos, you're on your own