If you’ve recently upgraded your Mac to OSX El Capitan and you have an issue where Virtualbox has decided to no longer work, try this:
for bin in VirtualBox VirtualBoxVM VBoxNetAdpCtl VBoxNetDHCP VBoxNetNAT VBoxHeadless; do
sudo chmod u+s "/Applications/VirtualBox.app/Contents/MacOS/${bin}"
done
Found via this link on StackOverflow.