As DebbieB said, Fantasmic is included in park admission. The dinner package she refers to lets you have dinner at certain park restaurants and get guaranteed seating for Fantasmic. If you don't do dinner package, you need to get in line for the show 60-90 minutes in advance to be assured a seat. With the dinner package, you don't need to show up that early, but the guaranteed seats are off to one side - good but not the best seats in the house. We've done it both ways. It depends on how much time you want to spend standing in line.
Steve