Basic Question

See if this helps. https://flyawaysimulation.com/knowledge/page/34/installing-addon-flight-simulator-downloads/ If not, come back here.

I (for instance) downloaded the Piper Arrow. I unzipped the package and put in into the Aircraft folder in fs9. In this folder (which is named pa28_180), I've got several folders - model, sound, texture, texture.1, .2, a file named 'aircraft.cfg', and a file named 'Piper_PA28_180.air. Of course, when I start FS2004, there is no Piper Arrow in the aircraft list. There is no question I'm doing something wrong, just trying to figure it out. Thanks for any and all help.

I have to restart my computer when ever I donwload an airplane. Try that.

The folder containing all of that has a name pa28_180, not called aircraft.
That folder goes into the main aircraft folder and you look under "piper".

I tried re-booting. No luck. Let me try another one. I downloaded the DeHavilland Dash 8, and unzipped the files into fs9 / aircraft. Within this 'aircraft' folder, there are the following folders - model, sound, texture, texture.1, and .2. Then there are two files - DH_Dash8 _100.air, and aircraft.cfg. That's all there is. So, am I missing somehting within this folder, or do I need to do something different to activate this plane within the program itself? Sorry for the seemingly inane questions - just haven't done this before.

First it would help if we could download the exact same folder.

The DeHavilland folder has to say that, not "aircraft", that's called a wrapper and can't be read by the menu.
DeHavilland can be spelled two or three ways so check them all.
You have 3 narrow windows in the aircraft menu. Put "any" in the top and scroll though the middle one. The bottom window will give you a choice of the two textures.