Am I suppose to rate the discussion that I have responded only?

If you can see the ratings buttons you're allowed to use them...doesn't matter where they are. Rate whatever you see. Other users discussions, responses, comments, photos...they are there to be rated. One thing though, 'rating' something does not necessarily mean using the + or -. Sometimes a post is neither particularly good or particularly bad, it's ok to not rate it...kind of like a 'neutral' rating. You know come to think of it, we really should have that as a button!