From X-Plane Wiki
Adding Scenery, Aircraft, and Plug-Ins
Adding Third-Party Aircraft
Perhaps the easiest place to find new aircraft is the X-Plane.org "Download Manager" page. All the planes in that subsection of the site are free, though X-Plane.org does have plane models (some of them very, very good) for sale.
When downloading a custom plane, it will typically be in a compressed folder (usually with a .zip extension) that contains the airplane and all its various paint jobs, airfoils, custom sounds, and instrument panels. Once the compressed folder is downloaded, users should be able to double-click on it to open or expand it on Macintosh, Windows, or Linux boxes.
From here, the folder can be expanded out into the Aircraft folder within X-Plane 9 directory, or the files within can be dragged and dropped into the Aircraft folder. Be sure to place the new aircraft files in a folder with the name of the aircraft—for instance, for a newly downloaded Piper J-3 Cub, the folder path in Windows might look like:
C:\Documents and Settings\User\Desktop\ X-Plane 9\Aircraft\Piper Cub\
With the new aircraft in the proper directory, open up X-Plane. Move the mouse to the top of the window (causing the menu to appear). Click Aircraft, then click Open Aircraft. Find the file there and double click on it to load.
Of course, users can also upload their own planes to X-Plane.org and similar sites. To do so, first create a custom airplane (using Plane-Maker) with airfoils, panels, sounds, etc. All the files making up the plane then need to be compressed into a ZIP folder to be uploaded to the Internet.
To compress a folder in Windows, right click on the file containing all the files needed for the plane, move the mouse down to Send To, then click “Compressed (zipped) Folder.” A new .zip file will appear in the directory.
On the Mac, control-click (that is, press the Ctrl key on the keyboard while clicking with the mouse) on the aircraft folder in the Finder, and choose "Create Archive" from the resulting popup menu to make a compressed ZIP archive of that plane.
These custom aircraft may be uploaded and shared at will. There are no copyright restrictions of any sort on planes made by users with Plane-Maker.
Adding Third-Party Scenery
Custom scenery packages, too, can be found on the "Download Manager" page of X-Plane.org, among other places. These may be downloaded and installed at will. Typically, custom scenery packages will need to be unzipped into the X-Plane 9\Resources\Custom Scenery folder. Additionally, the XAddonManager utility may be helpful for managing a large amount of custom scenery or downloaded objects.
Plug-ins are little programs that let the user modify X-Plane. People write plug-ins to do all sorts of interesting things like hang weights on the dashboard that move around accurately, run little tugs around to push your airplane on the ground, or draw interesting terrain visualization systems, among other things. Once again, X-Plane.org (and specifically the Downloads > Utilities page) is a good place to go to find various plug-ins and other things to tweak your copy of X-Plane.
For information on creating custom plug-ins, see the XSquawkBox site.