Mods are files created by the DEFCON community that alter the default scenario for DEFCON. You must be registered to use Mods.
To use Mods click on the MODS button in the main menu. Use the "Click here to download more mods" link to go to the Mods page on the Introversion website.
Download the mod file and place the folder containing the mod into the Hard Drive/Users/your_username/Library/Application Support/Defcon/mods folder and relaunch the game.
Click on the MODS button at the main menu. Select the mod and click on Activate then click on Apply and then close.