mip_matte_shadow for a white background?

[Dangertaz]

I'm trying to get this tutorial to work:

http://mentalraytips.blogspot.com/search?q=white

But my renders come out completely black or completely white. The objects are floating in space with the ground plane completely ignored.

I'm new to mip_matte_shadow; can someone walk me through how to set it up to get this "white room" effect? What does it plug into? What needs to be checked on/off


Any help would be much appreciated!

Thanks!!!