When you registered, was there any kind of verification code, math equation or anything you had to type in? I can't remember if there was when I signed up there, but many forums implement this to help block spambots etc, and I'm thinking they did have something like this.

If there was and you mistyped, it may have blocked your registration. Or if it didn't show up for some reason (say if you signed up via a mobile phone/tablet or through an app like Tapatalk (they use the Motorcycle.com version of it), it's possible the registration won't work through that.