If you're on Debian:
1) remove all installed package through Virtualbox Guest Additions ISO file:
sh /media/cdrom/VBoxLinuxAdditions.run uninstall
2) install Virtualbox packages:
apt-get install build-essential module-assistant virtualbox-guest-dkms virtualbox-guest-utils
Note that even with modprobe vboxsf returning nothing (so the module is correctly loaded), the vboxsf.so will call an executable named mount.vboxsf, which is provided by virtualbox-guest-utils. Ignoring this one will prevent you from understanding the real cause of the error.
strace mount /your-directory was a great help (No such file or directory on /sbin/mount.vboxsf).
