You should be activating Cruise by a switch on the stalk and a warning light will come on, drive to the speed you want and press the Set button, take your foot of the accelerator and the car will bowl along at the same speed. It will be de-activated if you press the brake or Accelerator pedal.
It is all explained in the Owners Handbook.
The passenger seat warning lamp is dependant on someone sitting in the seat and not plugging the seat belt in. If it seems the warning light never comes on then have a look at the wiring plug under the seat, it can be disturbed by rear passenger feet, cleaning under the seat, more so if you have a 3 door car. If the plug and wiring looks secure, just disconnect the plug and refit it, may just clear the problem.
