Some apps require monthly payments using a payment method of your choice. To view your current payment method go to Account (top right corner) > Payment Methods.
If your have selected a payment method, it will show the details on this screen. If you have not selected a payment method, the page will display a message informing you of this and show the "Add Payment Method" button.
Clicking the "Add Payment Method" button will redirect you to a secure page where you can add your credit card details.
You can add more than one card by clicking "Add Payment Method" again on the Payment Methods page. You also have the option of changing your default card at a later time.