First make sure that you are in your colleague’s planbook. To do this, click the Cc icon in the upper, left corner of the screen to go to the homescreen. Then go to your group and select the planbook of the colleague whose plans you want to comment on. Once you are in the planbook, hover over the card you want to leave a comment on, select the speech bubble icon and type out a comment. Finally, select "Add Comment" to save the comment. Your colleague will have access to the comment in a matter of seconds!
Note: If you are having trouble setting up a group, check out this article.