I dont think the K5000 was officially supported in CS6.
But you can easily put it in the "white list" yourself by finding the file called "raytracer_supported_cards.txt" in the AE Support Files folder
C:\Program Files\Adobe\Adobe After Effects C6\Support Files
Add the following in the file and save it again.
Quadro K5000
You maybe need to run the Notepad editor in Administrator mode (right click on Notepad and choose "Run as Administrator") to be able to save back into that folder, cant remember right now if thats the case.
Robert