If you install ubuntu as a guest on virtual box, you need to do a couple of things to install the virtualbox additions:
sudo apt-get install build-essential linux-headers-KERNELVERSION
sudo sh /media/cdrom/VBoxLinuxAdditions.run install
#Mais informações http://robotification.com/2007/06/15/how-to-install-virtual-box-additions-on-ubuntu/
#Caso o sistema da sua Virtual Machine seja Fedora acesse esse link http://www.sysprobs.com/install-fedora-15-virtualbox-steps-install-virtualbox-guest-additions-fedora-15
— Jackson Calhau 2011/09/05 16:38