Facing the desk, there's a small hallway to your right. Walking that way, you pass the entrance to the garden, the entrance to the observatory, and the guest elevator. At the end of the hall is the theater.
It's like any other theater... without the sticky floor and food in places where it shouldn't be. The walls are all covered with shelves filled with movies. From the soundless and black and white movies to ones that aven't even come out on DVD, there's a chance you'll find something at least somewhat interesting.
The theater's screen is also like the ones in regular movie theaters, while the seats are much more comfortable.