Ramo Auditorium and Baxter Lecture Hall are two auditoriums in the center of Caltech's campus. The two venues are in the same building, and are in fact exactly on top of each other (Ramo is on the ground floor, Baxter is 3 stories up). To get to Baxter, enter the lobby of Ramo and take either set of stairs up. Ramo seats almost 400 people, and Baxter seats almost 300 people.
The whole building is called "Baxter Hall" on the google map below. This is a large building, and you must go in the South entrance if you want to find the auditoriums. The building is one of two twin buildings (its twin is "Beckman Behavioral Biology") and they straddle a large grass field. To the North is the Beckman Auditorium, which is a good landmark (it's an unmistakable cylindrical white building that looks out-of-place).
See also this Caltech interactive map.