If you don't have a light in the scene that is emiting photons then GI in mental ray wont render anything different than a normal maya software render.

Try creating a spot light, aim it, then in the attribute editor for the spot light, select mental ray-- emit photon. play with the settings here and under the render settings for mental rays gi and causistics. This should fix your problem,

p.s. unless you really need it, don't active causitics it will only hurt your render times.