You will have to pay for it. It is a paid demo. Part of why they gave Aeris unique combat mechanics was to encourage people to pay for the Prologus demo. It isn't going to be free.
I just wish we would get it. Japan got theirs, what, 3 months before the game release? Here we are, 12 days away, still nothing.
Oh, and pay attention to your preorder bonuses! Preorder from Gamestop to get Cloud's KH outfit. Or preorder from Amazon to get Tifa's Amano artwork costume.