how to install virtualbox:
Add the following line to your /etc/apt/sources.list:
deb http://download.virtualbox.org/virtualbox/debian vivid contrib
According to your distribution, replace 'vivid' by 'utopic', 'trusty', 'raring', 'quantal', 'precise', 'lucid', 'jessie', 'wheezy', or 'squeeze'.
download and add The Oracle public key for apt-secure:
wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -
do as root:
apt-get update
apt-get install virtualbox-4.3
source: https://www.virtualbox.org/wiki/Linux_Downloads
p.s: I was prompted to insert my D7 CD
