|There is no username. Leave this field blank. The default password is admin.|
If this doesn't work, Web Utility Access via Wireless may be disabled. If so, you can only access the router menus via wired connection so you'll have to use an ethernet cable connected to one of the router ports.
If both of the above fail, then you'll have to reset the router to it's default settings by pressing the small reset button located on the back panel in for at least 5 seconds. Please note that this will not only reset the password to the default value of admin, but will also reset ALL other router settings to their default values, so you'll have to re-configure any settings you have changed.