If you're logged into the support forums and not into the plugins directory, then you'll see pretty much what you're seeing. The register link sends you to the support forums, where you're already logged in.
Usernames and passwords are shared across the support forums and the plugins directory and the theme directory and everything else ending with "wordpress.org". So if you're trying to log in with some other name or password, then it won't work. Given that you're logged in here, I suspect you typed the wrong password.
So, clear your cookies. Then log in using the top right box (or either box, really). Use the same username and password as you use here in the support forums.
The shared cookie thing is broken. We know this. We're working on it.