Don't assume that since you have ADRs at Hollywood & Vine, Brown Derby or Mama Melrose's that you will automatically get the Fantasmic Dinner Package. Apparently there are only a certain number of "Fantasmic Dining Packages" per day, and they are seemingly hard to get. I had an ADR for Hollywood & Vine for months, and had to call several times a week for 4 months until I finally nabbed an ADR for the FDP at Hollywood & Vine! We were there on December 6.
What happens is: Your server gives you a ticket when you have your dinner and they tell you to go to the gate near the "gas station" stroller rental at least a half an hour before the show. They let you in through a back area and you get to sit in a reserved seating area. From the way it looked, they had at least 1/3 of the seating set apart as Reserved. We had fantastic seats, right in the middle!