I tried as well when trying to use my Dotster user name. If it's a new Namewinner account I'd suggest just opening another. If it's an existing account, where you already have bids in see MarkMitford on the forum here. He works there.
I just received email from them about new $8.75 domains and one-click registration, so perhaps the changes they were making interfered with logging in.
Unless I'm way behind on the times and this is not as new as I think....