Turn the grey cap anticlockwise (when looking from below) and you'll hear the water flow/hiss, clockwise to retighten - it will just feel firm when retightening, not like a normal tap.
Regarding the comment if it goes to 2 bar, I noticed when filling, that my pressure gauge didn't move, then from .75 bar suddenly jumped to 2. This could be air compressing in the radiators or a slow gauge. But it's good at this point to bleed off air from the radiators - water should flow, not burble and hiss, when the tap is released. Keep it flowing if too much pressure is in the system, just sit a tall glass under the radiator plug to collect the flow. Don't let too much out or you'll dilute the additive.
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MarkD, June 2007