It's the middle icon (out of the three icons) below someone's profile. It will say ''add to this user's reputation'' you click it and then you can decide to either give green rep or red rep to someone + comment.
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Last edited by Nostalgia : 09-21-2012 at 07:28 PM.
On this subject, how do you know who has repped you?
Click your ''User CP'' and it shows all your recent reps and who they were from. The User CP is on the far right side on the page above your private messages.