The files of add-ons and mods for MM2 that you download are archives with the following file formats: .zip, .rar and .7z (this last is less used). Use a program called
WinRAR to be able to open them all.
After opened the file, extract the .ar file (that's the real add-on or mod for the game) in it to your main MM2 directory, that's usually C:/Program Files/Microsoft Games/Midtown Madness 2.