I'm using a IRF510 mosfet.
Now I'm completely confused as to how I should wire the thing up. I connected a PWM pin of the arduino to the gate of the MOSFET and a pull-down resistor to ground. The positive lead of the LED strip is connected to the +V connection of the power supply. One of the color channels of the LED strip is connected to the source of the mosfet. The drain of the mosfet is connected to ground.I also connected the ground of the arduino to the -V of the 12V power supply. (picture included as attachment, just imagine the single LED in the image is a LED strip)