There's a rocker switch below and to the right of the steering wheel, next to the headlamp levelling switch (if you don't have xenons) and the instrument brightness adjustment. One way is speed limiter, the other is cruise. You also have controls on the steering wheel to switch on and off and increase and decrease the speed.
There should be + and - signs on the steering wheel to the left and by the right side of the wheel about knee height there will be some switches.. If so, flick them and you should see a little green icon on the dash to say its active