It is a long shot but if the fan has been up a while and never had a light kit it is possible that at top where wires were originally wire to circuit the light wires were not attached. What I would do is check above motor. Coming out of top usually is a black, a white and a striped wire. For light kit to work the black wire and striped wire are to be twisted togather and connected to the black of circuit and white to white. With this connection both fan and light are controled on/off at wall switch with chain pulls used for light and speeds of fan.