I have searched through a lot of questions but it looks that nobody had the same problem.
I have successfully installed Ubuntu 12.10 on my MacBook (mid 2009) from a burned DVD. While keeping a 450 G partition for Mac OS X, I have created a main 150 G partition for Ubuntu, a 2G swap partition, and finally a small partition for bios-grub, since it didn't want to continue the installation if there were none. I remember having set the boot loader to the 150 G Ubuntu partition, but without really being sure of what I did.
When I restart my computer, the rEFIt menu gives me the choice between OS X and Ubuntu, but when I choose Ubuntu the only thing I get is a black screen with a blinking cursor... At least I didn't ruin my OS X, but I have tried tho reinstall it the same way and I got exactly the same result. Do you know what I could have done wrong ? Thank you for your help.