I've lived in PIttsburgh most of my life and have gone to the Carnegie zillions of times. I had never been to this cafe though. When you go in on of the entrances of the Museum of Art it will be in front of the large glass window facing Forbes Ave and the fountain. You can pick up sandwiches and cold foods from the cases or order hot food at the register. Their menu very pretty diverse so its good for all ages. Its cafeteria style unless you order hot. If you order hot food they bring it to the table. Meals are under $10.
My favorite item is their salmon cake with asparagus and wild rice. It was amazing! The best I ever had.