How to install the VirtualBox guest additions in a Debian VM
1) Open a terminal as root or su to root.
2) Update your APT database: # apt-get update
3) Install all latest updates: # apt-get upgrade
4) Install the needed packages: # apt-get install build-essentials module-assistant
5) Install kernel modules: # m-a prepare
6) Click on INSTALL GUEST ADDITIONS from the DEVICES menu (this will mount the guest editions image as a cdrom, otherwise # mount /media/cdrom)
7) Run # sh /media/cdrom/VBoxLinuxAdditions.run
8) Reboot and you should be ready to go.
Note: VirtualBox is a great (FREE) virtualization product by Oracle <https://www.virtualbox.org>