You'll need to create an account with Curse and then redeem the key. Beta Period: From Friday, February 7th at 12:00PM EST (4AM AEDT Saturday, thanks Hayness) until Monday, February 10th at …
The game release is on 16th October and will cost $25.
Curse are giving away free Beta keys which are no longer available (They were $50 until they stopped selling Beta keys).