All you have to do is place an order, choose free No-Rush Shipping at checkout, and your free credit will automatically be applied to your account once your order ships.
How to use your credit:
To use your credit, all you have to do is fill up your Prime Pantry box, and your credit will be applied at checkout. Please note that there is a limit of one $5.99 credit per one Prime Pantry order. Your free credit will expire April 30, 2016.