before upgrading virtualbox additions, one might need to uninstall the old version (this should actually happen automatically, when of new new version is run)

(actually this script will do it automatically for you, if you want to upgrade)
# run install script

if you can not find any (actually when new

try this:

# search for installed packages
dpkg -l|grep virtualbox

# delete installed packages
aptitude remove virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11 virtualbox-ose-guest-x11

# reboot

# mount VirtualBox Additions iso through guy
mkdir /virtualbox
cp -rv /media/cdrom0 /virtualbox

# run install script