The pipe on the bottom should go to your air intake pipework after the air filter. There's two types of BOV. This one is a recirculating type as it blows off back into the air intake. There are atmospheric ones that blow off into (as the name suggests) the atmosphere. These are the ones that the boy racers have fitted that make the stupid noise when they come off the throttle
Now I'm not saying anyone is 100% wrong here but do the 2 pipes go the other way round to as described in the posts above?
Top pipe (thin one) - vacuum line to intake manifold (or solenoids etc).
Bottom pipe (your missing one) - to inlet pipework after the turbo.
Middle pipe (one you can see to the left on the photos) - to inlet pipework before turbo/airbox.
Whenever I've seen dump valves before the pressure side of the connections is normally opposite the vacuum line as it's this pressure differential that dumps the boost. Only mentioning as it'll not do the engine much good connected the wrong way round.