You need to call and reserve your spots and then bring your vouchers along with you to the area. They also check your name off a list.
The CM we talked to on our trip in May 2002, said that alot of people just come with their vouchers and they didn't reserve. If there isn't room they are turned away. Others don't ever use this option - don't know that they have it or how to use it.
Also the area at MK for Fireworks viewing isn't good to see Spectro as well. So if you plan on seeing both, make sure your spot for the parade is very close to the reserved spot for fireworks viewing.
have a great trip!