I haven't play FO3 in a while and forgot how to add things like silencers and the clip add-ons to weapons. --- I think, therefore I am not Barry Burton. http://img213.imageshack.us/img213/4535/ohhaii.jpg
you find parts and click 'x' while you have the weapon selected and it will show you any compatible mods... it really sucks because i've been playing non stop since i got the game and have seen maybe 12-15 mods for tons of guns ive found, and 99% of the time the mod you find will be something dumb you don't want.
Hopefully mods get increased drop rate in patches or something because as such a highly touted feature of the game it is EXTREMELY lacking right now.