There is a CD available from Mercedesshop which will help.
But, the basics are:
Take a voltmeter and check to see if you are getting voltage to the glow plugs on the engine. With the engine cold, connect the voltmeter to ground and to the rear glow plug. Turn on the key and see if you are getting voltage to the plug. If you are getting voltage, but for a very short time, it could be the relay under the dash, or the sensor. There are two on the motor, the rear one is for the temperature guage, the front one is the glow plug sensor.
If you don't get voltage, follow the wire from the rear glow plug up to a plastic fuse box on the firewall behind the engine. There is a 50 Amp bar fuse in this box. If it is bad you will see it seperated.
I'm sure that Mercedesshop can supply you whatever you need.