Using the credits is very easy. Simply show the server your card and they will be able to apply the applicable credit to your bill with a quick swipe plus your signature.  To avoid any mishaps, it's a good idea to show them your Priority Pass card at the beginning of your visit so that they can verify your credits and guest credits. Sometimes these restaurants are dropped from the lounge network and it's possible your credit may not be available if you haven't kept tabs on these restaurant locations so it's good practice to verify it at the beginning.