I tried to put my plane in FSX by copying and pasting it into SimObjects/Planes folder, but when I start FSX the plane will not show up. I'm not sure why as the instructions said to simply paste it into that folder. It's a little frustrating.
If you're familiar with installing addon aircraft to previous versions of MSFS you should have no problems installing them to FSX. Installation of aircraft to FSX is basically the same as for FS9 or any other version of MSFS
except that the default Aircraft directory no longer exists. Install the aircraft to the FSX\SimObjects\airplanes folder instead.
Aircraft originally intended for FS9 might have the panel and/or sound aliased to FS9 default aircraft no longer included in FSX. For example; the b737_400. Aircraft without a panel will not be listed on the selection menu. It's a simple matter to fix this by re-aliasing Panel.cfg to a similar FSX default flyable aircraft like the b737_800. Providing they're installed correctly that should add them to the menu but I can't guarantee they will be fully compatible.
This is all covered in the FSX installation tute in the Simviation Help section.
http://www.simviation.com/fsxaircraftinstall.htm