Install Ubuntu on Yosemite I created a bootable thumb drive using the instructions here: How to create a bootable USB stick on OS X. I installed the 32bit and 64bit version.

I got a MacBook Pro (15" End 2011) Processor 2,2 GHz Intel Core i7 Storage 16 GB 1333 Mhz DDR3, Intel HD Graphics 3000 512 MB &amp; AMD Radeon HD 6750M, 128 GB SSD

When I boot the Mac and press alt I can select 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?
This may not be the answer you are looking for but I thought I'd boot it out there just in case. Assuming you have sufficient RAM (6 - 8GB) it might be easier to install Ubuntu (or any Linux dist.) in a virtual Machine. Parallels and VMWare Fusion are the two big commercial virtualization apps, but I have found that the free (and frequently updated) VirtualBox is easier than doing a dual boot system with the added benefits of it being quite easy to have both running at the same time.

As virtualbox is free you could try it and see if it suits your needs...