At the entrance to the Space Center. If you are planning for more than one visit, buy annual membership right away, it will be cheaper!
Magnificent Atlantis in full power! It amazes you how big this think actually is, at the same time - people spend months and months confined to this small place!
There are many exhibits on Atlantis before you actually get to the shuttle. If you are short on time, enter through the gift shop and you'll skip the long movies!