I fitted the same tensioner the other week, you activate it by pushing the end down on a clean hard surface so that the end comes out a bit further like something's been released inside. I put the tensioner into the hole in the cylinder head and rotated it so that it slotted in correctly (it sits a bit further in the hole), then pushed down on the back while tightening it by hand to engage the thread. Once partially in, I tightened it further using a 32mm socket.
My chain was nice and tight after, and I've had no problems since :thumbup: Hope this helps, maybe someone will be along later to explain it better! I think you'd know if the chain was slack, when my tensioner was fitted the chain wouldn't move very much if you pressed down on it with your finger. Try pushing the tensioner against something a bit harder? Having said that it didn't take loads of force either