Why are lamps and mesh lights not illuminating anything? There are several techniques to do a 3 point lighting system. 

How do you properly light a scene while using mesh planes as emission light sources? 

I'm following this tutorial, however despite setting up planes to act a lighting sources, the scene is still very dark. 




Actual Blender file here: 
https://dl.dropboxusercontent.com/u/460997/clock%20copy.blend
In your file, Clamp direct  and Clamp indirect have both been set to .03 in Render settings > Sampling.

This is forcing all the samples to return a value less than or equal to .03, which results in the apparent lack of light.

Set them both to zero to disable clamping:



Then it should work as expected: