VirtualBox – Effective UID is not root. Please try reinstalling VirtualBox.
References
http://superuser.com/questions/620936/virtualbox-does-not-start-on-mac-os-x
Only working solution on El Capitan after upgrading to VirtualBox v5, was
$ for bin in VirtualBox VirtualBoxVM VBoxNetAdpCtl VBoxNetDHCP VBoxNetNAT VBoxHeadless; do sudo chmod u+s “/Applications/VirtualBox.app/Contents/MacOS/${bin}”; done
