I just downloaded one of the add-ins for FS 2004 and have unzipped it.
Now I have no idea how to install it into FS 2004. The instructions are probably there somewhere but I'm not seeing them.
They're all a little different.
Most have you put the entire folder into the "aircraft" folder and some have you put part into the "effects", "gauges" or possible another. Your best bet is to unzip it again and read the "read-me" that came with it.
It opens slowly but it opens.