Im not sure if it is any thing like my series one, but I can try to help. there is probly a button on the center intrament panel to the left of the a/c control. It looks like a road with a little bridge over it. When you tern that on it should be orange. That activates the set and resume buttons.
If that does not work you may need to call the dealership because mine recently went out.
I have a 2000 disco. There is a button on the right side of the instruments (below the rear wiper switches) that turns the system on. To engage it, push UP on the SET button on the steering wheel. I took me a while to figure this out. The buttons are designed to operate with the fingertips from underneath, not with the thumbs from above. To disengage without hitting the brakes, push the RESUME button. This ,obviously, is how to resume speed as well. Good luck!