The below works with Debian 8.2 (codename "jessie") and VirtualBox 5.0.0.
From the Device menu of the VM window select Install Guest Additions CD image....
Execute the following as the user root:
apt-get install "linux-headers-$(uname -r)" module-assistant xserver-xorg xserver-xorg-core m-a prepare mount /dev/cdrom /mnt sh /mnt/VBoxLinuxAdditions.run umount /dev/cdrom
Vadim Penzin, September 8th, 2015
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