This is very strange. I'm running Debian and Ubuntu guests under the same virtualbox (hosted under Win7). And the Debian and Ubuntu mostly have the same packages installed. And I did the same steps to install guest additions. All went on fine.
Just that under my Ubuntu (Saucy), I can see the shared folder mount, but under my Debian (Sid), the shared folder mount is not there.
Any ideas? Thanks
mount -t vboxfs share mountpoint
?