Check the switch in the center (next to your hand brake) is plugged in. Mine wasnt when i got it. Its the brown plug not the black one.

Once you have that plugged in, test it. When your driving press the button, the light in it should light up. Now if you have the cruise stick on the steering wheel then you press down to set the cruise speed.

1, get the car to desired speed.
2, Press button in center.
3, tilt stick on steering wheel down.
4, let off accelerator, if the car doesn't slow then its working.

My switch is working, now i just gotta fix the stick xD