Hi, BD! According to the Walt Disney World website, "Disney Military Promotional Tickets may be purchased now through September 28, 2011 at participating U.S. military sales outlets." The tickets need to be purchased
before you get to the parks.
Once you get to ANY of the Walt Disney World theme parks (Magic Kingdom, Epcot, Hollywood Studios, or Animal Kingdom), you can activate them. The website says, "Eligible Service Members (or their spouses) who purchase Disney 4-Day Military Promotional Tickets for admission to the Walt Disney World Resort for themselves and their family members and friends will be required to activate those Tickets at Walt Disney World theme park ticket windows, no later than October 1, 2011, in order for those Tickets to be used. Valid military identification will be required."
I hope this is the information you need! Thanks for your question. Happy planning!