Go to: Login » My Account (top right corner) » Billing and Invoices Click on 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 front or 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.