[ubuntu] Installing on MBP, stuck on boot

Hi all, I was installing Ubuntu 12.04 on my MacBook Pro (model 5,3).. I made the partition with Disk Utility (16GB of free space) and I prepared an USB stick using this guide.. using rEFIt I boot with the USB until the Ubuntu’s boot menu, but when i choose the option "try ubuntu", it gets stuck with the black screen and the flashing white underscore..

so actually i can't move on, but i can't neither go back, because disk utility doesn't recognize anymore the free space partition that i created..

please, any help is appreciated..
thanks
DNL