Kostas1, from what I have seen your English is quite good. The vent/nipple is on the cam box which holds the cams which push the rods up to operate the valves on the engine. It is located in front of the cylinder on the crank shaft. On this picture of the engine:

http://www3.towerhobbies.com/cgi-bin...&I=LXAVCS&P=SM

the cam box is the round object with the two rods coming out. The vent/nipple is on the far side of the cam box and is therefore not visible in the picture.

Bruce