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wget -q http://download.virtualbox.org/virtualbox/debian/sun_vbox.asc -O- | sudo apt-key add -

Adding the repository:

. /etc/lsb-release

echo deb http://download.virtualbox.org/virtualbox/debian\ `lsb_release --short --codename` non-free | \

sudo tee /etc/apt/sources.list.d/virtualbox.list

Update the sources list:

sudo apt-get update

Source: VirtualBox Downloads