I was sent a confirmation Email, but it won't let me activate my account
Yes, unfortunately the page has an error You can not activate your account with the activation code. Vadim needs to activate your account and probably this is the reason why you can only play the demo version, because your account was not activated properly. So ask Vadim to do it before the time runs out. Otherwise, you will probably lose your premium status.
Your account is activated, have you finally found the email?