New pc same account cant install game. Thread

Thanks in advance to anyone who takes the time to respond. I recently built a new pc. I installed Desura and logged into my existing acount and saw that my game library was still there with a list of my games. I installed Arma 2 and Arma: Operation Arrowhead just fine with no request for the cd key and can play them at will. However, when i right click on Amnesia The Dark Descent and click install I am prompted to choose between buying it. installing the demo, or finding a current install on my system. I clicked on find current intall and tried all available options including entering my cd key and was told it was invalid. Does somebody know why i can install some games but not this one? If so please let me know why and how i can install my copy of Amnesia without having to spend the 20 dollars again. Thank you.