On a Visa or MasterCard, please turn your card over and look in the signature strip. You will find (either the entire 16-digit string of your card number, OR just the last 4 digits), followed by a space, followed by a 3-digit number. That 3-digit number is your Card Security Code. (See below)
This number is printed on your MasterCard & Visa cards in the signature area of the back of the card. (it is the last 3 digits AFTER the credit card number in the signature area of the card). If you cannot read your Card Security Code number, you will have to contact the card issuer.