Yes, I have read questions and answers with similar titles for this issue, which prevented me from installing Ubuntu for several MONTHS now, trying to figure it out.

I have a MacBookPro with triple partition (one for Mac Snow Leopard, one for Windows 7, one for Linux) created with reFIT firmware (not BootCamp). I set up the system according to these instructions for reFIT:

http://lifehacker.com/5531037/how-to-triple+boot-your-mac-with-windows-and-linux-no-boot-camp-required

Now. There is a free partition ready to accept Linux into its arms, but Linux does not want to participate.

Most answers to the issue "unable to find a medium containing a live filesystem" point to changing the BIOS booting system (which I don't know how to do, especially using this reFIT booting system), and to changing the socket of the USB (which does not concern me, since I am using a CD, actually I tried with a CD, then with a DVD, since a blank CD is only 700MB while the iso image file of Ubuntu is about 731MB).

Anyway. This is what happens:

I am in the Mac system (using the Mac partition) I insert the DVD with the burned image of Ubuntu (yes I have tried burning it again and agin on both CD and DVD blank discs). I restart the computer. When reFIT loads, I hold down the ALT key until the CD image appears. I select it, and hit Enter. A small Ubuntu icon appears at the bottom of the screen. Then a Ubuntu sign appears in the middle of the screen with small dots underneath, lighting up progressively over and over to indicate it's loading. Then everything turns black and the following message appears, at the end of a few lines of text: "unable to find medium with live file system".

Please provide very practical suggestions on what to do to an unexperienced wannabe Ubuntu very patient user.

Please start by saying how do I access the BIOS setup from reFIT bootup, and exactly what and why I need to try and change. (Will this mess up my reFIT bootup?)

And anything else I need to do to finally be able to install Ubuntu.

Thanks