When you download an addon it is probably a zip file or even a rar file.
These need to be unpacked with the appropriate application...........Winzip, WinAce.......WinRar........etc.
Once unpacked, look at the result. The addon is normally a .pbo, or several pbo's, which you then place in your OFP Addons folder.
But.......read any Readmes that came with the addon, these normally detail where to put everything that comes with the download.
Planck