I have had all situations happen.
Voucher accepted.
Voucher accepted at checkin, but did have to come back after activation and show activated pass. I was told if I did not, I would not get my discount.
Also have not been asked, but did recieve comment that it was in the computer that I have an AP.
You can not activate your pass at the International Gateway, you will have to go to Guest Services which is at the main entrance of the park. At the Yacht and Beach I would go to MGM by boat.
You can also go the Guest Services at Downtown Disney.