Steam is also saying my games are not installed even though they are. However I have not upgraded anything or changed anything on my computer. I am still running OSX Mavericks 10.9.4
On the odd occasion that it actually recognises that it is installed and on the even more odd occasion the game actually launches it then says "Damaged Content: Your download content is damaged and cannot be used. Please download it again" and quits. Nothing including a fresh install has solved this issue for me. I will never pay a single cent more to Steam!