You may not have support for CUDA applications out of the box - that is why the Retina iMac cannot work with Blender cycles - no CUDA since it is proprietary to Nvidia. This was the case for my son’s iMac although his is a generation older (late 2012) and the GPU is not very powerful.
The following quote is from Nvidia’s website:
New Release 6.5.46Supported MAC OS X
10.10.x
10.9.x
10.8.x
An alternative method to download the latest CUDA driver is within Mac OS environment. Access the latest driver through System Preferences > Other > CUDA. Click ‘Install CUDA Update’.
Try installing / updating the CUDA driver using System Preferences; if this does not work search for the correct driver.
Cheers
EDIT: Updated link (newer than one pointed to by gugusan so hopefully it works)