Did you get it from Verizon? If so, you probably have to call them and ask. Most likely it was changed when the Tech installed it and Verizon don't give you the password unless you ask because they prefer you don't change stuff. You could factory reset it, but then you would have to refill a bunch of DSL stuff, so I wouldn't suggest that unless you have done it before.
Only thing manual says is
»
www.dlink.com/-/media/Co ··· l_US.pdf4. Click Settings to configure your router. Once the login screen appears, enter Admin next to
Username and then enter your password (refer to login password on the label located on
the bottom of the Gateway). Click Continue.