The filling loop, usually is under the boiler. If I remember correctly two grey knobs turn anticlockwise to fill up. Clockwise to close.
Look under the boiler, there should be two grey coloured plastic knobs,
One right of centre and one to the left as you look at the boiler.
Open the one to the left first, Then the one in the middle, at which point, when you open, should start making a noise. Open it fully and you should see the presssure begin to raise.
Make sure there are no leaks any where. Make sure you close both afterwards.