If you are on the deluxe plan, you are entitled to both appetizer and dessert. On the basic, entree and dessert only (with very limited exceptions - family style all you can eat restaurants, the Fantasmic Dining package, and the pilot menu at Le Cellier include appetizers), and you may not substitute an appetizer for your dessert.
You can always order an appetizer if you are on the basic plan, but you'd probably have to pay cash for it.