the question is what you wanne dimm? in general you can use the pwn 0 - 3 (digital Output 9 / 10 / 11) to send diferent Voltage between 0V and +5V. SO you jsut need the right components to bring this singal on the right voltage and power.
If you just wanne controll LED or Lights with low voltage you could use a power transistors( i think there name was something like N-channel Power transistor).
If you want to control normal light (220 V up to 1kw) you have to bild your own little power unit. You can find a nice one at http://www.hoelscher-hi.de/hendrik/english/dimmer.htm
but take care, don't burn your self and use some protect switch (circuit breaker / FI Schutzschalter).
i hope i could help you