This modem/router seems to have some really helpful stuff in
its quick start guide. Page 16 has the address you put in your browser's address box-just hit Enter/Return and go. The default user name is "admin" (without quotes), and the default password is "password" (also without quotes). Once you've logged into the device, CHANGE THESE IMMEDIATELY. And of course, change them to something you write down-it's a pain to have to set up the whole thing all over again because you forgot the password. Oh, and
here's a link to the full manual. It may be for a slightly different version than your particular box, so you can look for the exact one
here using the part number as explained in the page.
It is much safer to configure ANY wireless router through an ethernet cable though. This precludes a problem where you change some setting and when it is confirmed, you lose your wireless connection. That's kind of scary.