How to use Boot Camp Assistant to create a dual OS X, Ubuntu system?
Personally, I would avoid using BootCamp altogether.  I would look at VirtualBox as a free solution to run Ubuntu on your Mac.  If it does not provide everything you require, you might try a demo of VMWare Fusion for OS X.  It will support things like KVM Hypervisor in a VM, which VBox will not.

If you insist on a native Ubuntu running on bare metal, just use Disk Manager to resize your system partition, add a second partition and install Ubuntu on the second partition.  You will likely clobber your apple boot record, but you should be able to restore that by using the Recovery Boot Partition and repairing your OS X installation.

After you have Ubuntu installed on a partition, try to use the boot menu to select the partition from which you wish to boot.  You get the boot menu by pressing and holding the option key after the boot sound and before the grey screen.  It will present a list of system devices that are detected.  It will also include USB drives, if that is a path you wish to take instead.  Select the one from which you want to boot, and hit enter.

That should/might work.  I haven't used a dual-boot Mac in some time as I use VMWare instead.