Additional modules can be purchased easily from the GameMaker: Studio Professional page. Once there, select the Modules you need (remove GameMaker: Studio Professional) and then click the "Checkout" button. After that all you have to do is fill in the required information. Once this has been done, clicking "Proceed" will take you to the purchase page to confirm the sale. Finally, once confirmed, you will be given the new license and an e-mail with it will be sent to the address you have given.
To use this Module, you must now use your YoYo Account to link your Module. We have a guide for this here.