ADD-ON Planes & Custom liveries not working/showing up.

Hello,
I recently bought FSX steam and after playing for some time, i tried to install other 3rd party Addon aircraft to FSX Steam. They dont show up (nothing at all) in my aircrafts. (These work perfectly with my friend's FSX Steam). I also tried installing extra liveries for the default 737-800.(AIR INDIA EXPRESS)
They too did not show up.
Please help.
Any Help is appreciated.
Regards,
Prabal