Put in a USB device, then do
dmesg | tail
Depending on what /dev/sdX device is listed, do
mkfs.vfat /dev/sdX -I
Note, the above command WILL wipe the USB drive!
Now take the downloaded .ISO, and do
dd of=/dev/sdX if=<path_to_.iso> bs=1M
Reboot the Mac, while holding Alt. A boot prompt should come up and allow you to boot from the USB.