In Blender The GPU-Compute button is greyed out. I have a fully updated system with a Nvidia GTX video card and the current version of blender. http://image.prntscr.com/image/9e80916076b944f8a5cc9c6ec78a1a99.png
Go to "File"->"User Preferences" and click on the "System" tab. On the left it should say "Cycles Compute Device". On my machine (I have the same card) when I change from "None" to "CUDA" the GPU Compute rendering option stops being greyed out. If that's not your case I don't really know what the problem could be. Good luck!
You have to make sure to not only select CUDA but also the correct video card. In the Settings in the "Cycles Compute Device" section it displays (if CUDA is selected) the available video cards. These appear to be able to be selected, so if one is selected the background gets darker. If I select my video card, the rendering device option inst grayed out anymore and the rendering process, if GPU Compute is selected, speeds up by a noticable amount.