The American exhibit area still houses a movie "The American Adventure", plus some exhibits (currently it's an exhibit on African American art). This is the brick building that's across from the lagoon. I've always thought of that building as the American Pavilion.
I think you might be talking about the outdoor stage that's across from the area I'm describing. I'm not sure what it's official name is. But that is the location of Candlelight Processional in December. They also host the "Eat to the Beat" concerts in October for Food & wine festival; plus the "Flower Power" concerts in April/May for Flower & Garden Festival. It's usually empty at other times.