UE4 keeps crashing when I try and start the editor

Hi,

I am new to unreal. When I downloaded a copy of the engine for my mac and installed it. It then asked me to start a new project file so I did. I then tried to open the new project file in the engine, and the screen filled with blocks sometimes coloured, then a error message appears. It happens every time I try and use the engine. I have uninstalled then reinstalled and that did nothing to remedy the problem. Please Help me.

27 inch iMac
OS X version 10.11.6
3.4 GHz Intel Core i7
32 GB 1600 MHz DDR3
NVIDIA GeForce GTX 680MX 2048 MB