You need to book a package through AAA in order to get the parking pass. You can't just show your membership card to park. We almost always book a package with them because we've gotten great rates. On our upcoming trip, our AAA price is $400 less than the price going through Disney, for the same package.
NOTE - it doesn't get you any discount on parking. What it does get you is parking in a special AAA lot that is closer to the entrance. There is no tram service from the AAA lot, so you will walk from there to the entrance.