Introversion's website is fully taxed, but if you get the demo you can just enter in an key into that and get the full game.
I want to love Defcon, but at the moment it isn't really doing anything for me. When the Defcon forum is back up I will see if I can get some tips. Not that I am doing badly, but it doesn't feel very strategic as to me the strategies all seem too obvious. I hope that I am missing something with the game's mechanics, but it isn't looking good.