Where is the cooling element in a fridge?

Cold air is denser that warm air, so putting it at the top allows the cold air to fall downwards, displacing any warmer air back towards the cooling element. It sort of maintains a certian degree of circulation and helps keep the temperature more even. If the cooling element was at the bottom, the cold air would just stagnate in the bottom, while warmer air might still be present near the top of the refrigerator.