When i try to share folder between host and guest (ubuntu server) in virtual box, i install guest additions, then i open terminal, and type
# mount -t vboxsf myshare /media/share
But i get error 'uknown filesystem type vboxsf', to solve this, type in the terminal
# apt-get install virtualbox-ose-guest-utils
After installation complete, type the mount again, and it worked for me.
# mount -t vboxsf myshare /media/share
Note: my shared folder name in host is 'myshare', same with the guest shared folder name