Hi Paul & welcome to O-F. Orbiter addons can be a bit tricky to install, especially if they are very old. First thing: make an OrbDL directory separate from your Orbiter directory tree, and put all your downloads in there, so you can quickly rebuild things. (It's not unusual to build a new Orbiter for a specific scenario, or to do a set of missions which need a specific set of installs, or to refresh things once in a while).
Can you list the URL's of everything you are downloading and trying to install, so I (and others) can have a look and see how to help.
Generally, addons come in 5 flavors:
1. ZIP file, where the top level of the .ZIP has Orbiter-like directories in it (e.g. Config, Modules). Example: D3D9ClientR15.zip. For this ... right-mouse the .zip file, Extract All..., and put in the name of your Orbiter root (e.g. C:\Orbiter), and let the un-zip put the files in the right places for you.
2. ZIP file with a .EXE inside. Example: PayloadManagerForOrbiter.zip. Unzip to a temp directory, and run the .EXE. If you just unpack the .ZIP into the Orbiter directory, it will NOT work.
3. RAR file, which needs a RAR unzipper like 7-zip. Example: SBB41B.rar. Have a look inside, and see if it's a standard Orbiter structure (if so - do similar to 1 above), or an embedded .EXE (if so - run .EXE as in 2 above). Almost always, a RAR file is a simple unzip into the Orbiter directory structure.
4. EXE file. Example: DeltaGliderIV-3_2010_20140109.exe. Just run the .EXE.
5. A .7z.001, .7z.002, .7z.003... file sequence. Example: Earth.7z.001 (hi-res textures). This is a multi-part zip. Use 7-Zip on just the .001 file, and it will access the other files as necessary to do the full extract.
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