This article is for the redemption of single year access GameMaker licences (Creator, Indie, Enterprise). This is not relevant for Subscriptions.
If you have been given a GameMaker single year access code, you can redeem it to claim your copy of the product from your YoYo Account page (if you do not have a YoYo Account then you will first need to make one - more details here). First you should log into your YoYo Account and then go to the Redeem section of the Dashboard:
Once here you should enter the code that you have been given in the Voucher Code section and then click the button marked Redeem (you will also need to tick the checkbox accepting the terms and conditions). This will will add the licence to your account and you can then go to the Products section to see them: