I suspect it cannot be done. The closest 'answer' I found was VirtualBox 4.3 Guest Additions on OSX guest but I would like to go with Ubuntu 14.04 guest on a iMac host. I have not been successful in installing ubuntu on my mid-2011 12,1 imac OS X 10.9.3 even after following several guides, but I can run 14.04 on that iMac. I have virtualbox 4.3.12 with the extension pack installed and Guest Additions on the Ubuntu 14.04 guest. Fading hope.