As soon as you have a user account for the wiki you can post comments.
You either can inline comment (a) by marking a passage of a text and clicking the pencil symbol. After you fillied in the form and clicked 'Save' the comment will be shown at the bottom of the page.
(a) Write a comment on a specific text passage.
Or (b) you use the option "Post new Comment" at the bottom of the page.
(b)