There are a couple of ways. You can click on Rate this user or xx points link next to the username in each message which will retrieve the page listing the feedbacks and ratings. On that page there is a link called Rate this user. When clicked, a remote window will be launched which will contain a form to rate that user.
Another method is to click on Ratings which will bring you to a page listing the index of users participating in user rating. The users are arranged alphabetically. By clicking on a letter, you can access a list of usernames beginning with that letter. Clicking on a username on the page will retrieve a list of feed backs and ratings that user has received. You can click on Rate this user link to launch the remote window containing the rating form.