Install Synaptic from a terminal using:
$ sudo apt-get install synapticOnce installed, run synaptic using:
$ sudo synapticSearch for the package virtualbox-guest-utils and select it for installation. It will a few other packages for installation, and after a reboot it will be fine.
You can get some more up to date utils by installing from the ISO image, but this requires installing some pre-requisites first.
You can find that info here:
You also need to prepare the system to build modules using:
$ sudo m-a prepareYou can also avoid rebuilding modules manually by installing dkms.