Install Ubuntu on Yosemite

I created a bootable usb using the instructions here http://ift.tt/1j0VtG9.

I installed the and 64bit version. I got a Mac book Pro 2011, MacBook Pro (15 Zoll, End 2011) Processor 2,2 GHz Intel Core i7 Storage 16 GB 1333 Mhz DDR3 Graphiccard Intel HD Graphics 3000 512 Mb AMD Radeon HD 6750M 128 GB SSD

When i boot yosemite and press alt i can select to boot from the usb drive.

Then it starts GRUB and i can select install Ubuntu.

Then black screen appears and the Ubuntu sound appears and then nothing happens.

What can i do?