There might already be a filling loop underneath, usually a braided pipe. If there is a braided pipe then at one end there should be a screw head. turn that until you can hear water filling up the boiler and then turn it off when the pressure gauge reads wht you want it too.
If you don't have a filler loop then I suggest you download the manual off the internet and see what needs to be done
Graham Underwood, March 2008