I know its near to the Greener Pastures Disposal site but i cant find it.
Could somebody show a picture or tell me where is it?

Image of the location taken from the wiki:
http://images.wikia.com/fallout/images/1/17/Alien_Crash_Site_loc.jpeg

If you have the Mothership Zeta DLC activated, you'll be transported to the mothership when you approach the ship.
Thus then you will not be able to pick up the weapon until completion of the DLC. Though it's possible to use Dogmeat
to retrieve the weapon for you when you are near the ship. Good luck

if you got that DLC, go to minefield, north of there is a power station. close to due north of there is the spaceship.

http://www.youtube.com/watch?v=_Jbjx9R5QPs

Youtube is your friend.

Go W (west from that disposal site).

You can get the firelance alien blaster (lights enemies on fire) from a random mini-encounter. Just wait till you hear a large boom in the sky. Mark the locale on your map, bring dogmeat there, and tell him to fetch a weapon. He will find either the regular blaster or the firelance. There will be alien power cells around too.

Thanks guys!