Technically you don't get free parking until after you have a valid AP. There have been instances when people haven't had to pay but the official policy is that you don't get free parking until you have an active AP.
Tell me what your AP voucher looks like and I can tell you where you can activate it.
Most time people don't have a voucher but a ticket that gets them into the park to get their picture taken for the AP. You enter the park and go to Vacation Services inside either park.
If it is indeed a true voucher, then you need to go to Guest Services Window before you enter the park. Guest Services windows are just outside both parks. They will give you a pass that gets you into the park. Once you are in the park, you just go to Vacation Services and get your picture taken and your card made.
If you let me know what your "voucher" looks like, I can tell you whether you should go to Guest Services or Vacation Services.