How do you top up the water pressure on a Vaillant VCW 242 EH?

I have located the cold water mains pipe, but cannot seem to get anything to happen when opening various valves. The guage is well below 0 at the moment, and there does not seem to be any water in the pipes. The kitchen taps appear to be working fine, so do not suspect mains supply issue. The boiler fan starts up etc, but the pilot light is not ingiting which I believe is due to the lack of pressure. Should I hear the water running in when topping up the system??? Please help!!