You can use your onward boarding pass to go to the lounge upon arrival. When you wish to leave the airport later, ask the lounge reception for a transit pass, you must have this silly slip of paper in order to go through security to the arrivals level. Then you can exit via immigration and customs. If you don't have that special pass (easily obtained in the lounge), you cannot go from the departure to the arrival level (and of course you cannot leave the airport from the departure level, only from arrivals).