Be sure to use the new login system, you can use it by clicking "play game" or typing in
http://feral-heart.com/login (http://feral-heart.com/login)
Use your username and password to sign in and keep it open as you play.
Adding onto what Likuu said, be sure to keep this tab open the whole time you are playing the game. Don't minimize the browser, keep it running in the background of your game.
Like said above, did you activate your account first? Like, did you go to your email to activate it? I had the same problem before. My account wasn't activated and due to the new log in system, I wasn't able to get in game. If this is the issue, wait until you see registration open again and activate your account. It wont work if it's closed.
Otherwise, use the link Shurtle gave 'ye. There should be lots more info there.
Hope your problem gets solved soon.