I followed this guide to install Ubuntu 12.04 on my Macbook Pro 8,2 (late 2011): https://help.ubuntu.com/community/MactelSupportTeam/AppleIntelInstallation

I used a CD.

I synced the partition table on rEFIt, and it went fine.

I do have an icon to boot on Linux, but when I launch it, after a few seconds I have "Missing Operating System" displayed, and that's all...

How can I fix that?

The only thing I see is, in the guide, it is mentioned this:

On the last dialog of the installer, be sure to click the “Advanced” button and choose to install the boot loader (grub) to your root Ubuntu partition, for example /dev/sda3. This will be the only partition with the EXT4 file system.

In Ubuntu 12.04 installation process, there is not such an option, but there is a dropdown menu to select where the grub bootloader should be installed. It was /dev/sda by default, but I selected my root Ubuntu partition (in my case, /dev/sda5). I got a warning message (but actually, it was the same warning message even when I selected /dev/sda), and I continued the installation...

Thanks in advance for your help!