Dude it's simple. Since you have it installed, go here:
Start Menu/YOUR COMPUTER USER NAME/Saved Games/Thirdwire/SF2 Game/Objects/Aircraft <--- Place aircraft folder only in here, in its entirety,
You may not have all the folders but depending on the aircraft download other folders will be present. If you have an "Objects" folder in the download, cut and paste in the "Start Menu/YOUR COMPUTER USER NAME/Saved Games/Thirdwire/SF2 Game/" folder. If you're still having issues figuring this out, do a search for "Thirdwire", and the folder should pop up in the search, go to it, explore and some of this stuff will hit you.
Which aircraft or is this just a general question? The reason I ask is because modders also create the folders necessary for you, so you just cut and paste and you should be good to go.