I am an FSX steam user and this is the first time downloading an add-on vehicle. I downloaded the Atlantis space shuttle here on FlyAway and did as the developer told me in their directions. But then they said to copy the entire Atlantis folder in FSX/simobjects/airplanes, which I didn't have in my files. Then I searched it up and I found it. It didn't work, however, and when I tried it again, the folder had mysteriously disappeared.
It won't disappear, if it did you wouldn't have any aircraft.
Check in Microsoft Games and any other main listing that has to do with your Flight Sim.
I don't have steam so I can't tell you other places to look.
Find the new manufacturers name and anything steam.