Water tap on Vokera boiler (circa 2001)?

Both the yellow and red lights are on (not the green one though), and I gather that I need to 'top up' the boiler by turning the tap beneath it. Sounds easy except there doesn't seem to be a tap as such. I've searched the bumpf that came with the boiler and it clearly shows a tap (looks a bit like a normal tap that I have on the kitchen sink); however there's no sign of anything like that underneath the boiler. There's a sort of black thingy on the right hand side. Is it that?

Please help!!