Inside mental ray check Enable Final Gathering, enable Auto Volume and that's it.
Kresimir
I do have both of these settings active and think this is why you are able to see it on the tea pot. I did check my scene closer and the back walls are over 75 feet away so I guess I can understand maybe why the fire light is not hitting them but the tea pot is 13 feet tall and the flames get to about 55 feet so I am not to sure why the fire light is not showing up at all on the ground with a flame that size.
I can see light from fire on the teapot handle - maybe you can remove teapot as it's obscuring the light that should hit the ground ?
Ok did a render wtih out tea pot and it does now show light on the ground but not very much. For a flame this size you would see a lot more fire light on the ground. Is there a way to up the power of the fire light?
or you can try turning up the mr FG multiplier in the fume dialog
I think i tried that and I did not see much of a change but I will look into it. Do you have any suggestions on a setting I start with?