Linux USB Creator for Mac is the easiest way to create a live Linux USB on OS X.
First, just select the Linux distribution you want to use. You can select a distribution from the list, or select a local ISO to use.
After selecting your chosen distribution, select a USB device to use.
The program will then download the ISO (or use the ISO selected), format the disk, copy the ISO and Install the boot loader.
Your USB Drive is now ready to use. Simply boot up your mac while holding the “alt” key and select the newly-formatted USB drive.