Here are some possible causes:
- You're working in blender internalBlender Internal rather than cycles:
- You're working with the compositing nodes:
Make sure that the selected option is the little ball, not the one that's selected in the image. - Your view is not in the right place. hitHit the Numpad . key on your number pad. ifIf nothing happens, hit A then Numpad ..
- Make sure use nodes is checked