[ubuntu] Ubuntu USB not booting on MBP5.1

Hello,

I've been trying to install Ubuntu 12.10 in the last 6 hours straight, I was trying to install it using the USB method which is guided by the official guide of Ubuntu.
I've followed every step and performed the exact steps to get it done, I've done it twice with two different USB sticks but whenever I restart I get no results as if there's a bootable USB beside the Mac HDD.

Then I've installed rEFit 0.14, whenever I do a restart I get the rEFit screen and the USB boot is there, but when I click on it all what I get is a black screen with a blinking cursor.

I'm using MacBook Pro 15" Late 2008 5,1.

Is there anyway I can do it??