To leave a comment on a piece of student work for a student to see, go to the Data page and select Quizzes or Writing from the headers listed. Click on the Expand Window graphic to the right of the quiz you want to comment on.
Click the Feedback link below the quiz score:
Type your comment into the field to the right of the student answer you want to leave feedback about. Hit enter to submit your comment.
After you submit your comment, there will be an option to request a revision from the student in the bottom right corner of the quiz. Checking this will add this revision task to their dashboard.
Note: Comments are private, so only that one student will be able to see the comment you post to their work.