That was my problem too. If you don't have a digit in your WAMU username, you will need to completely reregister and pick a new username and password. This worked for me. Good luck.
maybe they havent gotten around to your accounts yet! Be patient the message did say "On March 9 or after". They're not going to leave you behind!
When creating a username, they did tell me to use at least one digit.
But that's not the reason I couldn't log on, I don't think, because my original WaMu username already had a digit in it. In fact, when I tried to use that same username to register, it told me that the name was already in use.
I assume it was some glitch and I don't really care what it was. All I know is that some combination of "lost my password" and "register your account" worked. So if you can't gain access, just try that.