Hello! I have an AMD Radeon R9 290X GPU, After Effects CC, Windows 8.1 and i don't know how to activate GPU acceleration. Could you help me?
AMD GPUs cannot be used for the rytracer, if that's what you mean. They will only be available for fast Previews and OpenGL stuff.
Here's more info on the GPU in AE.
The only thing you really can't do is have GPU acceleration of the ray-traced 3d renderer.
But you have CC - not CS6! There's no reason to use that feature in CC. Use Cinema 4D Lite to create your content instead. It is faster, has more options, and is an all-around superior method for creating 3d in AE. And you can turn off the ray-traced renderer.