Can’t add nodes to compositor

Hello everybody!
I’m new to Blender, and I’m trying to follow this tutorial 3…
I’m up to the part about the “Volumetric background” part. I have done that part correctly and without problems (and I also understood what’s behind that, which is always a good thing :P).
Now I have to do the composting part.
The problem: I can’t add any node! Here’s a example screenshot of what’s going on in my Blender:


Every single menu is empty.
And the composting grid is empty too (there aren’t even input and output nodes).

If you need my .blend file to help me with this problem I will provide you with it

Thank you in advance and sorry for my English.

Uhm, I didn’t even notice that there was that option >.<
Thank you very much!