This page describes how to grade discussions that do not have a rubric associated with them. See Grading Discussions with Rubrics for help on that.
Go to the discussion you want to grade, and click the action arrow beside the name and choose “Assess Topic.”
This opens the window below where you can read posts, enter a grade, and write feedback.
Be sure you save the page.
See also: Grading Discussions with Rubrics.