You'll need to get into the router's configuration page. When you're connected to the router (use an Ethernet cable for this) go to start - run - type cmd - hit enter. Type ipconfig and hit enter. You'll see the gateway address there, type that into your web browser to access the configuration page. From there, you can log in and make the necessary changes. No CD is required, these are usually just some software that isn't that useful and maybe some scripted configuration, nothing earth shatteringly handy.
Ah...just seen you're on a Mac. I'm unsure of the procedure to get the gateway address from that, without just Googling it. The rest still applies though. Hope this helps!
P.S. see their support page
here for info on the configuration page, to see exactly what you'll need to change. I would recommend just setting it back to factory defaults and then setting it up yourself to your liking, avoids the risk of missing a setting or two.
