The online help topic for the review process is currently incomplete and confusing.
After a reviewer makes comments, you need to open the topic in review mode. The menu for topics within a document doesn't have Review, you have to go to the original topic in a folder.
In Review mode, you can reply to reviewers' comments.
In edit mode, you can view comments, by putting the cursor in an element with a comment and opening the Comment tab at right, but you can't add comments.