To create an Ubuntu USB installer with balenaEtcher: Open Etcher and click Select Image. Navigate to the Ubuntu disk image you downloaded and click Open. Insert your USB flash drive and Etcher should automatically select it. If it doesn't, click Select Target or Change to select the flash drive yourself.

Shut down your Mac, insert the installer USB stick and power on the machine while holding “option” or “alt” key until the boot menu appears. You should see something like this: Select EFI Boot and press enter. You will be presented with a GRUB bootloader menu: Select Install “Try Ubuntu without installing” and press enter.

Here's a recipe that works (at least, on my iMac with Ubuntu 14.04): Boot with bootable USB, install Ubuntu Partition your USB as follows: 200MB EFI boot partition 200MB ext2, mount to /boot Swap space if you want it One or more other partitions to hold Ubuntu proper (e. g. ext4 on /) Complete the install, reboot to OS X