Hello there! First of all which operating system are you using? If you are on XP the filepath should be My Documents/<User Name>/ThirdWire/StrikeFighters2Vietnam/Objects/Aircraft, While on vista you should find a Thirdwire folder on the Saved Games directory. Allways install the game to the default location suggested by the installation program, otherwise the location where your mod folders are supposed to bewon't show up. Now if I rememember correctly, in an unmodded install you'll only have the Objects folder (and another one that I can't remember) in your Saved Games directory, no folders for Flight, Terrains, Effects, etc. You will have to make them as you install mods on your game.
If you are new to Thirdwire Gen2 I would highly recomend to read this forum: SF2 Knowledge Base (Click Me) There are very useful guides in there that will help to get you started into addings mods to your game.
I hope this was useful.