I was using the node editor in an earlier project and I found it fairly easy to follow, so I opened up a new one to experiment. I added a plane, created a material and went to node editor, where I was completely dumbfounded. The nodes I expected to see with the diffuse and output weren't there, and the material changed to black. Not only that, but I couldn't even add any shader node!
You can't use material nodes in Blender Internal. Try changing your mode to "Cycles" in the drop down that has "Blender Render" in it right now.