Any 3rd party aircraft with AI in the description is not intended to be player flyable & will not normally display on the FSX free light selection menu.
All others should work fine, providing
you follow the installation instructions. This will get easier once you've successfully installed a few addons & you're familiar with how they're configured. You can always use the default aircraft for reference. Most of these have multiple variations in a single folder.
Also, make sure you have the 'Show all variations' box checked on the FSX Free Flight aircraft selection window. Otherwise only one variation of a specific aircraft type will be listed.
All installed player flyable aircraft aircraft will be listed on the selection menu under the manufacturer name, type & variation.
This can be identified by opening the Aircraft.cfg file in Notepad. For example, this is the first variation of the default B737-800;
title=Boeing 737-800 Paint1
description="One should hardly be surprised that the world's most prolific manufacturer of commercial aircraft is also the producer of the world's most popular jetliner. The 737 became the best-selling commercial jetliner worldwide when orders for it hit 1,831 in June 1987 (surpassing Boeing's own 727 as the previous champ). However, it wasn't always that way\s in the first few years of production, there were so few orders that Boeing considered canceling the program. They didn't, and the airplane has more than proven itself in over three decades of service."
Hope this helps.