This worked for me in Natty
Try installing virtualbox-guest-additions-iso from Software Center or Synaptic Package Manager. Then try again to install quest additions, and it should not ask you to download it, but open the Install Wizard.
This worked for me in Natty
Try installing virtualbox- guest-additions -iso from Software Center or Synaptic Package Manager. Then try again to install quest additions, and it should not ask you to download it, but open the Install Wizard.