Kampo

Hello there, I have bought a CD-Key for ArmA II via eBay equivalent, however there is no explanation how the game should be installed. I managed to install ArmA II Free, but that is not what I paid for. Is there any way to download the game client so that I can enter the cd-key? I would much appreciate any help, I've spent several hours now trying to solve this issue and I am on the edge. Yours truly, Kampo