If you have an old Beta key that you haven't registered yet, you will find that we don't allow registration directly via the client anymore. Please visit this URL to register your key and confirm your email address: https://accounts.cardhunter.com/register
Just got my email with the beta key, but the registration page doesn't load. I'm using Firefox on Windows 7.
Jon is in the middle of deploying a new build, which updates the web server that hosts the registration system. Give it a few minutes and it should be back up.