I have seen people add a comment but I can't see that link anywhere in my questions.
Is there a need for a certain amount of reputation to comment?
You need at least 50 reputation in order to add comments to any post. However, until then, you should always be able to post comments on your own questions and answers, and any answers to your own questions.
The otion should be "add comment" and should be located in the bottom left of any eligible post, as shown below using your question here on Meta.
under "Leave Comments" (and be sure to read the footnote, too!)