1. Log into the Customer Control Panel with your main account's username and password.
2. Click the "Account" Tab.
3. Click on the "Payment Methods" link.
4. Click the "Add Credit or Debit Card" link.
5. Enter in your credit card information and click the "Save" button.
Make sure the card is enabled for Auto Payments so any subscription that is setup for automatic renewal will bill against the correct card by default.