Using a mobile device or PC/MAC, there should be a link starting with https:// posted by
@jbuck1975. Go ahead and click that using a web browser such as Safari, Firefox, or Internet explorer. It should open a website much like the one you are browsing. That is how you use it.
Ha, in all seriousness, what exactly don't you understand? The instructions are right there on the link. What part do you not understand? Forget password the PIN after inputting the incorrect one a few times or click forgot password (don't have Wii U in front of me) and it'll pop out some 10 digits. Input that into the link. Enter that in Wii U.
or you know, maybe ask this random "jerk" what he input the password as or try 6969, 1111, 0000, 1234, 4321 etc. I don't honestly believe random people would enter a passcode like that, this seems more for people who genuinely forgot the password, was sold the console, the console was stolen, or for those kids who want to bypass the parental controls their parents set for them.
Post back here if you need further instruction. I'm sure someone may be able to go into more depth for you, or you can search the plethora of threads on here discussing this exact topic at hand using the search box function of this forum.