After downloading the ISO, use Disk Utility to partition your USB device. Ensure that you've selected Options -> MBR . After formatting the disk...
1) Ensure that you've marked the partition active
fdisk -e /dev/rdisk1
2) Download syslinux and write the critical mbr binary your device.
dd conv=notrunc bs=440 count=1 if=mbr.bin of=/dev/rdisk1
After that, just use UNetbootin as usual.