When you register for the game you only receive an account for the game itself. You will need to register for the forum separately. When registering you will need to use your game credentials (username and password) in order to create your forum account.
For this to work, you will need to have a validated email address in the game. If you haven't already, you can validate your email by going to the game's settings, then to 'Email and Password', from where you can request a new validation email. Upon validating your email address, you are awarded 50 Diamonds to use in the game as well.
Once your email account is validated, you can register your forum account using one of those pages depending on which English server are you playing on:
If this didn't help, feel free to contact us and we will be happy to look into your issue!