I'd need to see a manual, as these things sometimes have different names.
However, in general terms, if you don't know/have changed the access password and forgotten it, you can reset your router back to it's defaults and set it up again.
Doing that normally involves turning off the router and holding down the recessed reset pin while you turn it on. Keep the pin depressed for 30 seconds.
I've been reliably informed that if you haven't changed the username/password (or you have just set it back to the factory defaults) you can access the menus by using the factory default username "administrator" and leaving the password box empty.
Warning: You will need to know your adsl username and password, as provided by your ISP if you plan to do this, as that will be deleted too.