[MaxWilson]

XLegend,

Mods are regular old text files with the .dm extension. Calchet may be right if the filetype is a zipped or rar'ed compression of the mod (often done if the mod comes with a special graphic identifying it or graphics for new units), but even after you extract it Windows won't do anything with the mod file(s). Instead, save the mod to the mods subdirectory of your dom3 installation (e.g. c:\Program Files\dominions3\mods) and then start Dom3. You enable mods under "Game Options" on the main screen, before you start/load a game.

-Max

P.S. If you want to understand modding better, read the documents in the docs subdirectory of your dom3 installation.