Copy the downloaded aircraft yellow folders into: AIRCRAFT main Folder
Location - Sim Objects - within FSX
Example : 737-200 / copy the entire folder / into the "Aircraft" Main Folder within FSX
PAINTS ONLY
Example: COPY and PASTE only the "TEXTURE" folder into the Main folder for that Aircraft only.
then add / COPY - the CFG File Entry that came WITH the Aircraft that you downloaded and click save in the upper left hand corner of the Aircrafts CFG File, and close.
ALSO - IMPORTANT - Any & All photos / Pictures that come with any download , some do, some don't - will need to be "Re-Named" with the word "Thumbnail" and saved into a JPEG File format using a simple photo editor like Windows Paint.
That will Allow Screen Shot "Thumbnails" to appear in your Main Flight Simulator Menu, when you start up Flight Simulator,, you'll have a Tiny Thumbnail Photo appearing of the Aircraft / Helicopter / Ship in the Aircraft Flight Menu for you to see and chose from.
If nesessary try researching some of this on YOU TUBE, for more information.
A fairly Decent kinda Tutorial on Injecting Aircraft into Flight Sim :
Link :
http://www.youtube.com/watch?v=V-Ol9KxdPaI