My Macbook Air comes with Intel HD Graphic 4000 card, so in order to take advantage of GPU, I will install OpenCV with OpenCL support. (no CUDA for my Macbook Air)
I would use Mac Port to install opencv.
If you have installed OpenCV with Mac Port before, then uninstall it in terminal by
sudo port uninstall opencv
To install OpenCV with OpenCL support, first edit the port file for opencv
sudo port edit --editor nano opencv
This will bring up an editor with opencv port file. Look for the line written:
-DWITH_OPENCL=OFF
and Change it to
-DWITH_OPENCL=ON
Then use Mac Port to install OpenCV + OpenCL:
sudo port install -d opencv +opencl
If there is no error, the installation should be done.