How to install VirtualBox in Ubuntu Gutsy

sudo apt-get install virtualbox-ose virtualbox-ose-source
sudo module-assistant prepare
sudo module-assistant update
sudo module-assistant unpack virtualbox-ose
sudo module-assistant auto-install virtualbox-ose

to verify the module was installed:
$ ls -l /dev/vboxdrv
crw-rw---- 1 root vboxusers 10, 62 2007-10-08 13:27 /dev/vboxdrv

sudo rmmod vboxdrv
sudo modprobe vboxdrv

sudo gpasswd -a vboxusers

add vboxdrv to /etc/modules

Source: https://help.ubuntu.com/community/VirtualBox