I have installed ask ubuntu lens on my 12.04 desktop some time back and I could see it in my installed applications from Ubuntu Software centre. But I cannot find it on the dash. How I can launch it ? I installed Alacarte hoping that I can see it there, but could not find it there either. Note: I reviewed the question here, but could not find answer to my specific question.
Assuming you have logged in after installing the package (as I'm sure James has), you can simply open up the Dash by clicking on the Dock button on the top left of the screen. The dash will open and there will be an icon of a cartoon balloon with "Ask" in it. Mine's in the middle of the set of icons. Each of these icons represent a lens.
Click in it and type "How to launch askubuntu lens". This very question will be in the penultimate row, in Answered Questions (unless you accept my or someone else's answer in which case it will move up a row to "Questions with Accepted Answers").
If you don't see the icon there I'd say there is a bug somewhere.