I seem to have tried and clicked all available options but just can't fathom out how to reply to a comment. I've never encountered a more confusing user interface (and I used to work on FRTRAN in the 80's with punch-cards).
There's the added confusion of the security (I'm not a robot) entry and a Preview button then a Save button that overlays a Reply button and lots of New Comment and Subject buttons. What on earth is going on with this page, I just want to reply to a comment.
To reply to a comment, you type in your comment in the big comment box, copy the captcha text (taking care with the capital letters etc) then click save.
You might well have replied successfully already, but the comment won't have been shown straight away as comments currently have to be approved by an administrator because at one stage there was quite a lot of spam on the forum.