You need at least 50 rep before you can add a comment. (Although you are always able to comment on your own questions & answers.) Once you have 50 rep, an "add comment" link will appear underneath each question and answer.

I see you answered a bunch of questions quite recently, You'll get 50 rep pretty quickly once people start voting on them. 

[See the FAQ for more details][1]


  [1]: https://askubuntu.com/faq