I have exactly the same problem. The folder in /Steam/SteamApps/common/Defcon only contains the soundtrack folder with corresponding soundtrack files.
Verification of game cache does not solve the problem.
Defcon is not available yet, it seems. Which is a pity, as it is one of the games I had bought ages ago but not got around to playing yet.
Same here, Kubuntu 12.10 x64. If the game doesn't work yet, why can it be listed and downloaded under the Linux section? Makes no sense. Seems more likely it's a bug to me, given that Introversion's other kit works fine.
I also have the same problem :<
I just bought the game DEFCON, it is in the list of games to download.
But impossible to play the game files does not download.
When I click on "play" an error that says "the exectuable does not exist"