As of May 2010, Walmart no longer sells Skype prepaid cards, but you can always purchase Skype Credit online.
If you have purchased a prepaid card and haven’t used it yet, you can still redeem your prepaid card. To redeem your prepaid card:
When you enter the unique PIN (a 19 digit number) found on the back of your card, make sure you leave no spaces between the numbers.
If you have a Skype Unlimited prepaid card, you will need to redeem the card, and then contact Skype Customer Support. The Skype Unlimited prepaid cards no longer work so you will be given a Skype Credit voucher instead.