mate, that is the bleed valve- the little cross-headed "screw" on the TOP of your radiator, next to the reservoir cap, i assume??..
and its supposed to be SCREWED IN.... its NOT suppose to be open in normal operations, otherwise, the water's gonna run out... maybe that explains why u're losing "steam" ie. water..
top it off (50-50 mix of coolant/water), tighten the screw down..(as with most plastic screws, dun overtighten, or u;'ll brak something!)..
hope this helps..
cheers!