How can I dim a 10w LED?
Well, it appears that your driver should do that for you. Since you already have the MBI6651 driver, read its datasheet and so you can get a guess on how it should work:


attach a 9-35V supply to the Vin-GND terminals. I suggest to use at least 12V
attach the LED to the terminals LED+ and LED- (remember the polarity!)
attach a transistor to pull down the pin DIM
send a PWM signal (frequency between 100Hz and 1000Hz) to the transistor and you should see the LED light up and then dim


If it works then you can try to send to DIM directly th PWM signal, but personally I don't recommend so (unless the board already have the transistor)

EDIT:

How to attach an nMOS to the DIM pin. Since the pin is internally pulled up, you don't need any pullup. So you just need an nMOS. I suggest you to use a SOT-23 logic level nMOS connected this way:



no external components (as long as the nMOS supports 5V on the gate, but every discrete mosfet should). As for the nMOS, I prefer the SMD case because it is easy to solder also on perfboards (just use three pads forming an L shape and you'll see that you can easily solder it). Just choose any n-channel mosfet with a maximum threshold voltage of 2V or less and you are done