Found this at WDWIG hope it helps.
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. NOTE: There have been recent reports that you can now purchase a new AP and activate the voucher at any WDW 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.