Upper right hand corner of the message, just below "Display Modes" is a little balancing scale symbol. Click on that nd yu get a little box where you are allowed to leave a brief comment on the person's statement. "Agree" adds to a person's Rep points, and "Disagree" takes away from them.
As an alternative, or if you want to leave a longer comment, in the lower right corner, click on "Quote" and that pens up a new message space. You can leave as long a comment there as you want to.