Where do I find the "Security code" to test "Commerce PayPal WPP Credit Card" with my "sandbox buyer account"
Just put in any number that matches the card type - e.g. 111 or Visa / Mastercard / Discover or 1111 for American Express. As far as I know, the PayPal API will accept any value. It should just be the card number and expiration date that have to match your sandbox test card details.