You need to install the virtual box extension pack available from the download section at www.virtualbox.org
However opening the downloaded extension pack from virtualbox and clicking the install button, sometimes gives the following error
"Failed to install the Extension Pack Oracle_VM_VirtualBox_Extension_Pack-.vbox-extpack. The installer failed with exit code 127: The value for the SHELL variable was not found the /etc/shells file This incident has been reported.. Result Code: NS_ERROR_FAILURE (0x80004005) Component: ExtPackManager Interface: IExtPackManager "
I find that installing the extension pack using the command line fixes this problem.
simply cd into the folder were the extension pack you downloaded resides,
and type the following. replacing the xxxx with the version of the extension pack you downloaded.
sudo VBoxManage extpack install --replace Oracle_VM_VirtualBox_Extension_Pack-4.1.xxxx.vbox-extpack
and it will install properly, you should get the following back.
Successfully installed "Oracle VM VirtualBox Extension Pack".