When you hit the 'ON' button, does the Cruise Control indicator on the
instrument panel come on?  Does the horn work?  If no to both, I'd suspect a
bad/damaged clockspring in the steering wheel.

If the cruise indicator comes on, but just won't activate, I'd suspect a
problem with the cruise control servo or cable.  Check the cable between the
cruise control servo (should be under the driver's side battery tray) and
the APPS module.  Make sure it's not busted.  Then check the vacuum lines
between the cruise servo and the vacuum pump, on the back side of the power
steering pump.  You need vacuum to actuate the cruise control.  (This is all
assuming you have an automatic transimssion...  manuals handle the cruise
control differently).

Another possibility is a faulty brake pedal switch...  if the computer
thinks the brake pedal is depressed, cruise cannot activate.