From a Howto in the VirtualBox forums:
After installing the Guest Additions,
you can use the Shared Folders
Functionality. To access them, you
have to define at least one in the VM
settings. Open the VM Settings and go
to Shared Folders. You can define them
there. Click on the Add button and
browse for a folder you want to share.
Make sure that the name of the share
doesn't contain any illegal characters
like white spaces. Keep the name as
simple as possible. In this howto, I
will use the name share.
Now that you have defined a SF, it's
time to mount it. Boot the Guest and
open a terminal. Create a folder where
you will mount it on. E.g. in your
home folder. I will use ~/host for the
mount point. Now mount it with the
following command:
sudo mount -t vboxsf share ~/host
Note that with this, the default mount
options are used and all files are
owned by root. This can be changed by
adding some mount options. Options are
passed on with the -o parameter. You
can use multiple options with one
parameter, seperate the values with a
comma. See the man page of mount for
more info on which options you can
use. The User Manual also notes the
options compatible with the Shared
Folders. To mount the SF so that you
are the owner of the files, use this
command:
sudo mount -t vboxsf -o uid=1000,gid=1000 share ~/host
If you want to have it mount
automatically upon each boot, put the
mount command in /etc/rc.local (Debian
based distro's), or whatever script is
run at the end of the boot process.
The Shared Folders service should
mount them automatically, but that
doesn't always happen. Using
/etc/fstab has little effect, because
that file is processed before the SF
module is loaded and will fail to
mount the share. Sometimes, the share
does get mounted because the GA check
for it when they are loaded upon boot,
but it's very flaky, meaning it
doesn't work most of the time. You're
better of with the first option. When
you put the mount command in
/etc/rc.local, so it's mounted at
startup, you can't use the short
notation for your home folder. During
startup, everything is done through
the root user, so using ~ for home,
means it's the home folder of Root
(/root). Change it to the full path.
For example:
mount -t vboxsf share /home/<username>/host