From the X-Plane user manual:
When downloading a custom aircraft, it will typically be in a compressed folder (usually a ZIP file) that contains the airplane and all its various paint jobs, airfoils, custom sounds, and instrument panels. Once the compressed folder is downloaded, you should be able to double-click on it to open or expand it.
From here, the folder can be expanded out into the Aircraft folder within X‑Plane 10 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 this:
C:\Documents and Settings\User\Desktop\X-Plane 10\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 click on it to load.