From the dashboard, select 'Billing' from the left hand menu.
If you already have a credit card on your account, it will be displayed here. You can have up to 5 credit cards on your account at any time. Select '+ Add Credit Card'. The below box should popup where you can add your card number and expiry date. Once you have added the details, click on 'Save Card'.
A green confirmation box as shown below will be displayed if the card has been successfully added.