I have the same setup, OSX El Cap and have it working. Did you follow the install of the serial driver?
One issue I had, so I bring it up is that of the cable. I was using a 'power only' cable for a while. A different cable worked. Also, I seem to have trouble with the USB port on the left side. I have no issues with the one on the right. I think they are both USB3.0, so I am not sure what the issue could be.