Everything is rendered black, when I set it to be a gradient Hi I'm trying to render my file to be a gradient (cubes) and the floor to be glossy black.  I was following this tutorial on youtube (without the wireframe effect) and I can't figure out why this is happening.  The tutorial is https://www.youtube.com/watch?v=0r5b8LaniMQ

You can see my blender file here: http://www.pasteall.org/blend/38973
The gradient material isn't assigned to any of the cubes; they just have a default diffuse material:



To assign your gradient material to all of them:


Select all the cubes (I used border select (B) to do this)
Set the material of the active object in Properties > Materials
Press &#9096; CtrlL> Materials to link the active object's materials to the selected objects.