permission in FTP ... help...

ok i want a forum on my website and this forum is php

all forums with php needs to be set and i have to go into smartFTP and put the permission to 666 or rw-rw-rw.

BUT when i do that it says Permission denied and i m logged in as sub user of that site(also tried with the head user but didn't work either)

if you could tell me how it comes i don't have permission to change it then i would be helped very much.

also it allready says 644 ( owner : read , write. public : read. guest : read. )
just to let you know.