You want to download the one with the highest number. I think that is 143 at the moment but Im not sure. If you check the stickies the thread will be called "MCO Version ###" and all the downloads will be there.
Find the version with the highest number in that thread and download it. Also in that thread is a link to Microsoft Visual C++ 2010 (x86) redistributable which you will also need to download and install. after installing both of those the game will run correctly.
If you plan on making new maps or some other stuff (I don't know b/c I don't do it) you will need to download XNABuildSetup.exe. That file can be found in the same thread.
If you would like to play some non default campaigns go to the MCO campaigns thread and download the campaigns it will download an installer. Install the campaigns in your default MCO directory and you can then play the new campaign.
Enjoy