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 & 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?