quick

i have a PSU and when i plug it in... nothing happens!? the ps fan doesnt even start up. i am forsure this is a good PSU cause i just went and bought a new one and itdoes the same thing. when i plug it into an old mobo of mine the little green light lights up so i know there is power going through it... am i really just retarded or is there something else i need? i need a quick reply as im getting ready to go out of town for a few days and i wanna finish this thng up!! HELP pleeease!!