Every comment starts with the username of its author. By clicking on a username, you get to the user's personal page. From there you can email the user.
Every comment ends with ADD or MORE. MORE means there's more to see--either someone has commented on the comment, or the author wrote a two-part comment and only the first part is displayed. Click on ADD (or MORE) if you want to comment on the comment; it will take you to the comment's main page.
The most useful link at the top of the page is the Add link. Click on that to add a comment to the page. You can add comments on a phrase, or at the end of the page. Clicking on Edit will let you view the raw text of the page (and save changes, if you know the password).
In addition to the Add and Edit links, you'll also find a Top link (which takes you back to the home page) and _usually_ a Back link. The Back link takes you to the page where the comment was first added. If you reached the comment from a different link, clicking Back will not take you back to the page where the link was.