I don't know how to reply to a comment
Until you start typing one letter in comment box the post button would be in hidden condition.
Just simply type, whatever you wanna reply to comment and click the post comment.
If you are not type, the post button would be in dead mode/Hidden mode.
See, When i start typing its automatically glows and after you finish typing just simply click post comment.
If you want to notify a specific person than use @HIS NAME there before you comment or Send a Private message to him.
send private message to specific.