There is no username required to log in.
The default Belkin router password is simply (blank) -- do not enter anything in the field, leave it blank. Once you see the login screen, simply click Submit without entering anything.
Resetting the router password
If this doesn't work, it means that someone has changed the default password without telling you. If this is the case, simply reset your Belkin router to factory defaults.
You can do this by holding the reset button on the back of the unit, using a paper clip or pen, for 15 seconds until the router automatically reboots itself.
Now that you're logged in, it's a good idea to change the default admin password to something more secure. You can find this option in the control panel, or might be prompted (on certain routers) to set one immediately upon logging in.
You're all done!