To the left of every post is the box that has the author's name and avatar picture and some statistical info. At the bottom of that box is a row of little icons. One of them looks like this: . When you're looking at a post that you think is worthy of an additional rep point for the author (or that you believe is so bad it is worth taking away a rep point)....click on that button and it will bring up a little pop-up box where you choose to leave a positive or negative rep point (it is positive by default) and you fill in a reason why you're giving the point.