For some reason (at least as of this writing), the official Ubuntu documentation on creating a bootable USB of Ubuntu for Macs is now recommending using UNetbootin. I haven't found that to actually work. I dug up a cached copy of the old instructions (which do work), but I wanted to document the old procedure here, just in case the cached copy later becomes inaccessible.
Here are the steps to get a bootable-to-Mac USB of Ubuntu
Launch up /Applications/Utilities/Terminal.app.
In the terminal put in
You may notice that your target.img ends up turning into target.img.dmg, which is totally fine.
Plug the USB stick in and run
Unmount the USB (without unplugging it physically) by running this command
Copy the .img (or .img.dmg) file to your USB using dd:
Then remove the USB drive from your Mac and put it in the Mac you want to boot from. Hold down the option key while booting up, and then select EFI as the drive to boot from.