To add a comment on a discussion, go to the left-hand menu and click Discussions, you will then see recently added discussions, recent replies, all discussions and create a new topic.
Click on the title of the discussion you would like to comment on and it will then open and you will see the full topic and all replies. You can respond to comments by clicking reply under the comment you would like to comment on, if you would like to comment on the initial topic enter your comment in the text box under Your Comments and click submit.
You also have the ability to upvote a comment on the right-hand side of each comment made.