I got a download to work! I downloaded this Piper Cherokee 180 several days ago, https://flyawaysimulation.com/downloads/files/4629/fsx-piper-pa28-180-cherokee/ thought I was following the instructions correctly, but it wasn't showing up in my aircraft selections. I don't have it working perfectly. Several guages aren't functioning, but I'm happy to say what I figured out for anyone still having downloaded aircraft trouble.
This was my route to my list of airplanes:
Microsoft Flight Simulator X,
My mistake: putting the whole PA28-5 file in that list.
I had to open the PA28-5 file, copy the Microsoft PA28_180 file, go back to the list of airplanes, paste the file I'd copied to that list. Now it's available for me to play.
I'd already copied the guages file from the PA28-5 file and pasted it in to the Microsoft Flight Sim X Guages file. It just took until nearly midnight last night to realize what I needed to do to the airplane file.
I found this forum last night during my search for solutions. I don't know if anyone else who comes here searching could be helped by the same flash of insight, but I wanted to report, just in case.
My husband and I own a 1963 Cherokee 180. That is why I wanted one in my FSX, so I could practice cross countries and play with a virtual plane similar to our own. Somewhere in our house is my copy of FS2004. I have FS2002 as well. In 2004 I had a bunch of cool airplanes downloaded, but had forgotten how. Now I think I'll download a new fleet.
There is SimObjects in between Microsoft FlightSimulator X and Airplanes.
I just downloaded an Air Tractor. Let's see if I did it correctly.
It works! Yay!