The Node Editor won't work for the object selected I'm still learning, but I'm trying to follow a tutorial to apply texture to a model.
I only want to apply the texture to one object in the scene, so I selected it, unwrapped it, and painted over it.
Now I selected it in the Edit Mode (I can't select it in the Object mode because all is locked together) and went into Node Editor, but the nodes are greyed out. 
Why?
Make sure Use Nodes is enabled in the header of the node editor or in the properties editor: