You can buy games with Refereum here: 

Once you have chosen the game you want to buy, click on Buy Now

Your MetaMask wallet will pop-up (if you don't have MetaMask installed and/or an account, you can see the steps on how to get started with MetaMask here). Once you're logged in you will see this window: 

Type in the gas fee and the price and then confirm. 

That's it!