Without burning a CD, Unetbootin for Mac lets you create bootable Live USB drives for Ubuntu and other Linux distributions. You can either allow UNetbootin to download one of the many out-of-the-box supported distributions for you, or supply your own ISO Linux file. A bootable Live USB drive may be created by UNetbootin for macOS. It can load distributions by downloading an ISO file for you or by using an ISO file that you have already downloaded. Select the ISO file or the distribution that you want to import and then select the destination drive and reboot it.
If it does not turn up on your USB drive, reformat it to FAT32. If you have used the installation mode 'USB Drive', boot from the USB drive after rebooting. On Macs, this normally includes pressing a button like Esc or F12 immediately after turning on your device, while on Macs, before booting OSX, you should keep the Option key. If you have used the "Hard Disk" installation mode, select the entry from the Mac Boot Menu after restarting. Download and open UNetbootin, then select the option "disk image" and provide it with an ISO.