You click on the little pair of scales between the green online button and the warn sign you can at the bottom of peoples posts.
Hover over the scales, and it says "Add to X's reputation" -click that and a popup will appear. Click either I Approve or I Disapprove, leave a reason, then click "Add to reputation"
To find out more, go to The Pok?Community Technical Institute section of PC -there's a reputation FAQ in there.