Heres a really easy way, which i do to get that effect ...

Click on object then click on its initialShadingGroup in its attribute.
Then click on the mentalray to bring out the info.
Under Custom shaders click on the checked box next to Material Shader.
Then scroll down to find Mib_amb_occlusion.
When you click on that, that will load everything.
All you gotta do now is just go into your render settings and change Render Using which default is "Maya Software" and change it to mental ray and put it on production quality or whatever you like ...

hope this helps