Your key also provides you with access to any export modules you have purchased. When purchasing a module, your key will be updated and you will need to enter the updated key into GameMaker for your new features to take effect.
To do this, GameMaker has an Update License option under the Help drop-down button as shown in the screenshot given here:
Clicking on this will show a dialog where you may enter your new license key. You will then be prompted to restart GameMaker: Studio to finish the process. Once that is done, you should have access to your purchased features and modules.