After a first attempt to post I am trying again. I want to install Ubuntu 12.04 on my Macbook Pro running yet Mavericks. As my super drive is defaulting I use a bootable USB stick.
At least I found 3 methods and threads to do so. At this time what I did is installing refit (after installing refind) , so I am able to select the boot disk at the very beginning of start-up. I have created the bootable usb using Unetbootin. So now I am able to see the Pinguin at the very right of the screen when I turn my laptop on. I select it and it leads me to a blue square inside a black one with "Default" as only option. I select at it gives me this image!
From there nothing happens so far. The laptop begins to overheat and I have to force a shutdown.
Meanwhile I moved forward as in the beginning I was not able to boot from the Usb stick (I cannot see the Pinguin when I turned my laptop on)
thanks for your help