The AAA Lounge in the MK is indeed on the far side of the Carousel of Progress.
If you follow the curve of the building past where the qeue enters the theater there is a small gate with a very small AAA logo on it. You follow a pathway around to the back of the building (out of guests' view) and there is a glass door leading into the lounge.
There is a AAA staff member there to greet you and help you if you need anything. There are some seats and a TV playing a Disney movie. The highlight is getting a free can of soda and getting "away" from the crowds.
My wife and I visited in February and since we booked throught AAA figured we'd stop in and check it out. When we went we wereonly the second people to come in that day (based on the sign-in sheet) and I think the AAA rep was happy to have some company.
It was also interesting to be able to go freely somewhere that seemed a little backstage.