We were at the Luau a few weeks ago. Call Disney Dining for a reservation. They will hold the reservation with a credit card. You must pick up the tickets at a Disney resort customer service desk. When you pick up the tickets, your credit card will be charged for the Luau.
I believe there was a 48-hour cancellation policy, but you can double check when you make the reservation. I believe the discount we received was 10% from the Disney Club Card.
Have fun and enjoy the luau.