Normally PWM is used when you want to dim a DC load. In your case you are trying to dim an AC load, this is done very differently. In short AC dimming requires a zero cross sense, and an interrupt timer.

For more detail on how AC dimming works check out this website:
http://www.dv-fansler.com/Computer%20Lights/dimmers.htm

NOTE: You can't dim inductive loads with this method, only resistive loads like light bulbs.