Something which often gets overlooked by filter and aquarium manufacturers is the finer details on how things should be set up. There is no right or wrong answer to the question of which direction your filter outlet should point however I would strongly advise that the best direction to point it in initially will be towards the surface of the water.
This should be done in such a way that you cannot quite hear the sound of the water moving but that you can see the surface being agitated. There is a science behind this and it all comes down to the oxidisation of the aquarium water. Fish require enough oxygen in the water to help them to breath. The more surface water movement there is, the more oxygen can transfer in to the water, therefore if you have the filter outlet pointing towards the surface of the water it helps this process along.
You can of course point it under the surface to produce under currents but only if you are quite under-stocked on number of fish or have something else to agitate the surface such as an air stone or powerhead running.