Do you have another computer that is currently running the game? That's what I do when I want to play it on my laptop- I start it on my desktop and stream it onto my laptop

What happens when you log into your steam account on the new PC and go to your library? Usually when then game is installed you see a fairly large green "Play" if the game is installed , if it isn't i'd imagine your see a blue "install" button in it's place.

Clic the arrow next to Stream, you'll have the option to play locally or to stream.

WOOT !!! got it thanks megagab