Be sure you are in the Teach mod

Be sure you are in the Teach mode when you access the discussion (otherwise it will not show). Select a name from the list.
Scroll down to the bottom of the dialog box with your colleague's entry. At the bottom right you will see a button "Review This Post." Click on that.
It comes with a comment box, but that box has a limited size. If you have lengthy comments you will not want to enter them there. When you are done marking the Grading Form (WebCT for rubric), exit the form. At the bottom of the dialog box containing your colleague's posting, on the LEFT this time, you will see a Comment button. Enter length comments there.