Go to Admin Panel > Modules > Widget Management... Scroll down and find the "Top Posters" widget. Drag and drop it any on any of the sides you want it to show up. Click "Save"
Display forum widgets : This must be 'yes'. Width column 1 (left) : Minimum value = 180 to show up and 0 to hide it. Width column 3 (right) : Minimum value = 180 to show up and 0 to hide it.