The button goes two ways, its next to the TCS/ASR button. Near your right knee bottom of dashboard (if the car is right hand drive)
You push it one way for cruise control (green light on dash), middle position is off, and the other way is for the speed limiter. (orange light on dash)
The speed limiter function (on mine anyway?) dosnt completly keep you under the speed you set, just I reach the limited speed, and If i keep my foot planted, it slowly continues to acclerate.
Hope this helps