When you shop at Walmart, you need to present it to the merchant and then you select the "credit" payment option and sign the sale receipt.
Walmart $500 gift card cannot be used in Apple’s App Store. In order to use on iPhone you need an iTunes or AppStore Gift Card.
The card is accepted in the fifty (50) states of the United States and the District of Columbia, excluding Puerto Rico. You can not use it at any merchant, including Internet and mail or telephone order merchants.