Configuration: Virtualbox 3.2.6. Host – Windows XP, Guest – Debian Lenny
Mount VBoxGuestAdditions.iso in guest window. If you can`t see content of mounted CD then:
debian:~# mount /media/cdrom
Be prepare and update system:
debian:~# aptitude update
debian:~# aptitude upgrade
And the final:
debian:~# sh /media/cdrom/VBoxLinuxAdditions-x86.run