Leaking water, frustrating!!?

Hi, We have just fitted a dishawasher using one of those valves that make their own hole in the water pipe (as recommended). I can't stop the begger leaking now. It's not a huge amount but enought to drip every minute or so. Is there a knack of tightening them? I worried about overdoing it and squashin g the pipe. Any idfeas?

Masonmou
December 2009