I've heard others report differently, but when we activated ours at Christmas time we had to pay to park. We were told to take our parking stub along with our vouchers to the redemption location. When we were actually given our annual passes, we would be refunded what we paid for parking.
It went off without a hitch.
FWIW, at Christmas when we showed our APs for free parking, the attendants hardly looked at them. (A less than 1 second glance and then they waved us through.) On trips since then, I've noticed them taking more time to read the AP to make sure it is valid.