To install a mod, follow the directions above.
To apply an installed mod once you have DEFCON running so you can play it, follow these... (see the animated pic)
Start DEFCON
Go to Mods in Main Menu
Select Mod
Click Activate
Click Apply
Go back to Main Menu
Start New Game.
Mods listed in RED mean they have a map that isn't the same as the default map and others that join must be using that same mod. Mods in white mean anybody can join or you can join any other server while using that mod.