There should be a "Filling Loop" on the pipework below the boiler, it is a silver braided metal hose with a quarter turn valve at one, or sometimes both, ends. The valve will have a black or blue handle to turn it on. If it has two valves, open one fully, and the other slowly until your pressure gauge starts to move up, shutting the valve(s) when the needle reaches 1.5 bar.
scott, December 2008