Go to: Login » My Account (top right corner) » Billing and Invoices
Click on the Add Credit Card button under Billing Information.
You need to enter the following details of your credit card:
Security code is the 3 or 4 digit number available either on the front or the back of your credit card depending on the card type (Amex, Visa, Mastercard).
Security code for Visa / Mastercard: 3-digit number located on the back side of the card.
Security code for American Express: 4-digit number located on the front side of the card.