Howtogeek posted a tutorial about getting shared folders in an Ubuntu 11.04 virtual machine in VirtualBox to work

Have you installed Ubuntu 11.04 as a virtual machine in VirtualBox but have had problems getting the Shared Folders feature to work? We were able to add a shared folder, but were unable to access it.
We found the secret to accessing shared folders in an Ubuntu 11.04 virtual machine in VirtualBox. VirtualBox adds a user group called vboxsf to the Ubuntu 11.04 virtual machine. Any user that needs to have access to shared folders defined for that virtual machine need to be a member of that group. This article shows you how to add a user to the vboxsf group.

  Access Shared Folders in a VirtualBox Ubuntu 11.04 Virtual Machine