First thing - this thread should be moved to hardware right??
I reckon the fan should rotate so that it blows cool air into the box forcing the warmer air out of the vents. The actual direction of rotation will depend on which way the blades are set. But this should be easy to test.
If you think it's blowing the wrong way turn the fan around.
Reversing the polarity of the fan may turn the blades in the opposite direction but this could reduce the effciency of the fan or simply damage it electrically.