The stable version AskUbuntu lens in available from the Software Centre for Ubuntu 11.10 and 12.04: https://apps.ubuntu.com/cat/applications/unity-lens-askubuntu/

The latest development version may be installed by adding the AskUbuntu Tools PPA to your repositories with the following commands:

  sudo add-apt-repository ppa:askubuntu-tools/ppa
  sudo apt-get update
  sudo apt-get install unity-lens-help unity-scope-askubuntu unity-scope-manpages unity-scope-sumo unity-scope-asklibreoffice

Then log out & log back in.