I need help installing add-ons. I originally believed and it had worked before that you just copy the aircraft file into the simobjects then aircraft folder when adding an add-on. However I have been trying several times to add some new aircraft but when I open FSX after doing that above procedure the new aircraft dont show up. I am doing something wrong, any help is apperciated!

Each addon is installed in its own way. You need to find instructions on how to install the particular addon.

I do the instructions that come with it. I copy it into the aircraft folder but when I start the game it doesnt show up.