Why doesn't my dimmer switch work after installing LED bulbs in my ceiling light?
This is common with older dimmers and LED's. Old dimmers use a small amount of power to activate the switch (almost always silicon controlled switch called an SCR or thyristor) the small amount of power is enough to light the LED's. 
You need a dimmer that is LED compatible I would get a quality name brand dimmer as not all dimmers are created equally.