When first posted in 2013, this question never received an answer, because there is no answer. In other words, Intel Macs can not BIOS boot from flash drives. I make the BIOS boot assumption based the the following phase from the OP's question:
"UEFI removed in order to be able to boot on Macs"
If the OP assumes the UEFI boot method has been removed, then the only remaining boot method must be by using a BIOS. Let me explain this further.
When Apple started building Intel Macs with 64 bit processors, most (if not all) models contained firmware with 32 bit EFIs. This prevented the EFI booting of 64 bit operation systems (including OS X). However, these models could BIOS boot 64 bit operating systems. This led to the creation of special Ubuntu installation ISO files designed to install a BIOS booting GRUB. These files can be identified by names ending with the sequence -amd64+mac.iso. The correct procedure was to burn the ISO file to a DVD and use the DVD to install a 64 bit Ubuntu on an Intel Mac. Eventually, some users are still installing Ubuntu this way. For example, see the question Booting 64-bit Ubuntu Images on a First-Generation Mac Pro 1,1.
Unfortunately, you can not transfer these types of ISO files to USB flash drives and successfully boot using an Intel Mac. Basically, you can not BIOS boot Intel Macs from a USB port. However, you can install a BIOS booting Ubuntu as a dual OS X/Ubuntu boot. This can be accomplished without using a DVD or USB flash drive. For a example, see the question Installing Ubuntu without DVD or Flash Drive.
Both bmike (in a comment) and kenorb (in a answer) have correctly pointed out that Ubuntu can be installed using a USB flash drive. In both cases, the flash drive is used to install Ubuntu for a EFI boot. As I stated earlier, I do not believe the OP wanted an EFI install of Ubuntu.
Best Answer
Open Disk Utility (you can find it in the Utilities folder). Insert your USB drive, and select it in the left pane. Choose the tab "Restore" on the right side of the window, drag the image file into the "Source" field and the USB drive into the "Destination" field. Click the "Restore" button.
That should do it.