Like martin has said you need to remove the bust sensor from the transfer box by lifting the car to get access.
The sensor will either be a 19mm bolt if the car is an early turbo or a 13mm if the car is a later model..
In order to get the system working again you will need to fit a working sensor in place of the bust one as without a working sensor, the system will not come back on..
Once you have another sensor fitted in place then you remove the bolt next to it that is in the mount.(
be warned the sensors are very easily snapped off when tightening up so be carefull)Then you fill with the oil from the breather pipe that comes out of the top of the transfer bx and should be coming out up by the header tank.
( this must be done on level ground) This is done using a small funnel and is a slow proccess..
You stop filling the oil when the oil starts to flow out of the mount bolt hole..
Then you replace the bolt..
If you intend to get the 4x4 working again you will need a replacement sensor which can prove difficult if you have the earlier turbo model as they are not made anymore..
Thing is, once the earlier temp sensor fails it is ruined and cannot be re used so a 2nd hand one will have to come of a working 4x4 box..
The later sensor is a simple switch so a 2nd hand one can be used again..

Check this page for more info..
http://www.cav16.co.uk/transfer/