before upgrading virtualbox additions, you need to uninstall the old version:

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

if you can not find any

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