You have to select both the manual and the rating - there are two drop downs. Then the grading drop down menu comes up on the discussion post.
I think Moodle really Broke something that was not broken - they made grading the forums three times longer and harder than the old board grading system - where you could see all ONE students posts.
In making this system where you have to go into each individual post - it creates a disconnect with the student and the posts they leave. Also there used to be a space to give feedback.... that too is gone.
They went three steps backwards on the Advanced Forums.