It's not exactly easy to find.
You have to park on a nearby residential street, or take one of the many MUNI lines that lead to the Ocean Campus of the City College of San Francisco (or hoof it a half mile from BART). Walk onto campus through the main street of Phelan Ave. Once you pass the science building, take a right against a one-way street. Go up a small hill, walk through a quad on your left, and enter the non-descript theater. Finally, standing in the lobby, you'll get to revel in Diego Rivera's 74-foot Pan-American Unity Mural, a staggering work of artistic genius, and a gem of the San Francisco arts.