Click in "Add a comment"

You don't reply to others' comments-at-the-1st-place. If they leave a comment, that means they don't want to be posted afterwards, otherwise they'd choose to answer the question or correct your "entries", doing which they can have you the chance to "comment". Although it sounds like s/he just wants to "comment" other than "correct/answer", they wouldn't get any notification if let's say, you do want to like, "discuss" with them, by leaving them comments on your own threads. Most of them wouldn't care to check back.

Click "Add a comment" link.

Just add your own comment.