Look at all the pipes coming into the boiler, one of them is the mains water inlet. It should have a short length of flexible pipe branching off of it and entering another pipe going to the boiler. This flexible pipe should have a tap or valve that you turn on for a few seconds - you may need a screwdriver to operate the valve. As you turn it on watch the pressure gauge and turn off the valve when the pressure gets to about 2-Bar. Hope this helps.
Ian
December 2010