This system is controlled through the Teves Mk60 ABS controller as far as I'm aware, if the light is on the first thing to try is to 'reset' pressures. To do this inflate all tyres to the desired pressure and then put ignition 'on' and then press and hold the button under the handbrake for 5 seconds....this should then set the system again.
Failing that it could be a case of checking the ABS system for DTC's.