Desura is experiencing some issues with their servers that is preventing installation. We only know that they are working on it, but don't have an ETA - hopefully we will have more information later this afternoon (will post here).
In the mean-time if you've purchased the game but can't install it, please send us a PM and we'll make sure you can get a copy until Desura is fixed.
thanks (and sorry)!
UPDATE: according to our tests this issue has been resolved, we're still waiting for a confirmation, but we're pretty sure things are good to go again.