VirtualBox, problem mounting shared folder

[Colin Law]

I hope someone can help with mounting shared folders in VirtualBox
(4.1.2).  I am using this for the first time, running a Precise
virtual machine in Ubuntu 11.10 host.  I have installed the Guest
Additions which seem to be working ok except that I cannot get shared
folders to work.
In the guest os I added the user to group vboxsf.
In Devices > Shared Folders I added a share with path /home/colinl
(which is on the host) named it host_colinl, checked Auto mount and
Make Permanent.
I re-booted the guest.
I see that it has correctly created the folder /media/sf_host_colinl
but it shows as empty (even with sudo) and in nautilus it does not
show the mounted icon on the folder.
I removed the share from Devices > Shared Folders and added a new one
with a different path and share name downloads.  This time I did not
select auto mount.
In a terminal in the guest I created /media/host_downloads then
sudo mount -t vboxsf downloads /media/host_downloads
and it says
/sbin/mount.vboxsf: mounting failed with the error: No such device.

dmesg shows nothing at that point, nor does syslog.

Google has not helped me.  Any suggestions gratefully received.

Colin