Tell us exactly what you are trying to do and what you are experiencing. There is nothing that you need to do to "enable" that GPU for the OpenGL features of After Effects.
See this page for details of the GPU features in After Effects: http://blogs.adobe.com/aftereffects/2012/05/gpu-cuda-opengl-features-in-after-effects-cs6. html
Well I have a Intel HD Graphics 4000 graphics card on my macbook pro. Apparently it is and isn't supported by after effects (some sources say yes others say no). So if it is how do I enable GPU because it is gray. If it isn't apparently you can add it manually in the raytracer_supported_cards.txt file. What is the exact thing I would add if I was to add it manually?
As I said, there is nothing that you need to do to enable the OpenGL features. They just work automatically.
Regarding the CUDA feature, the ray-traced 3D renderer: That feature requires CUDA, which is a hardware architecture that your GPU simply does not have. It is an Nvidia technology that is only on Nvidia GPUs.
This is all explained in the page that I lined to in my previous post.
However, you should not concern yourself with the ray-traced 3D renderer. The ray-traced 3D renderer is an obsolete feature that has been superseded by several better ways of doing 3D in After Effects, including using Cinema 4D (which is included with After Effects now):