You can do it by buying it directly or by upgrading from a trial or demo version of the game. When your order is completed you will get an email with all the information you need. This email contains your license key to unlock your game.
If you did not download and install the free trial yet, you can download your game via the downloadlink in your email, or download it via the Free Download option on the website.
After you installed your game, there is one thing left to do; enter your licensekey in your game and you have unlocked the full version!
Please save your license key (or the email containing it) in a safe location. If you decide to reinstall the game, you will be able to use it again.
Note: PC and Mac versions of one and the same game are considered as two different products. Before you proceed to checkout, please make sure you’ve chosen the version compatible with your OS.