Its been a little while since we did the package. I am not sure if you can book in advance, I don't think so. The morning we arrived at the park, we went to one of the windows at the front of the park. Asked the CM were we had to go to make Fantasmic dinner ressies and they directed us to were we needed to be. We did the Brown Derby resturant, very pricey but very good. I believe we did lunch. Anyways, after we had our meal, the waiter stamped a card stating we had showed up for the meal. At the show you are directed to a different area of seating. We were right up front and had a great view. We didn't have to wait, just walked right in.
The two resturants that were available for ressies were Mama Mel Rosas and Brown Derby. Mama Mel Rosas books up quick, its a little cheaper than Brown Derby.
Hope this helps!