I tried to make a Lubuntu 13.04 LiveUSB with UNetbootin (Mac version), but it is not bootable. Not on the PC I want to boot it, and not on the Mac I made the drive with (On Mac it just doesn't show up in the bootable drives menu (alt during startup) and on the PC I want to boot it it says "Missing operating system"). The drive is formatted as FAT (via Disk Utility).
What is going wrong?