According to allearsnet.com:
"To activate the voucher, turn the voucher and a photo ID into the Guest Relations window at either the Magic Kingdom, Epcot (either front or International Gateway side), Disney-MGM Studios, the Animal Kingdom, or either Guest Relations office at Downtown Disney. You cannot activate your voucher at a WDW resort Guest Services. You can now purchase a new AP or activate the voucher at any WDW major park ticket location, not just at Guest Relations. This may be a good option if you're doing this at off peak times of the day. It may still be a good bet to use Guest Relations during the park opening rush.
TIP - There is only one place off property where you can exchange your voucher. That is at the Ocala Information Center in Ocala, Florida. This is run by Disney's Guest Relations. They can also make resort reservations for you there.
TIP - The Guest Relations Window at Epcot's World Showcase is generally MUCH less crowded than other Park Guest Relations windows
A word of caution: There have been reports of people turning in their vouchers and getting activated AP's with the wrong expiration date on them. The passes will have the date you bought the voucher rather than the date you turned it in. Always check your passes for accuracy before you walk away from the Guest Relations window. It will avoid major problems for you later on."
