Fallout 3: how do i start alien quest?

The crash site is a secondary location that does not appear on the map. Travel past the factory North of mine field. Next travel to a destroyed bridge. Then head Northwest until you encounter a pocket of radiation. If you have trouble look for a destroyed house with a unidentified craft.

I had that problem with that particular addon as well. All I did was uninstall it and then reinstall it - I know, lame. Once I did that however, the quest started up just fine and I spent the next few hours happily blasting creepy little aliens and stealing their treasures.

go to the alien crash site where u get tha alien blaster

follow link. it shows a map of fallout 3 world and where things are

http://fallout.wikia.com/wiki/Fallout_3_map