First, convert the ISO to a bootable DMG using hdiutil:
hdiutil convert -format UDRW -o bootable bootable.iso
This produces a file called “bootable.dmg”.
diskutil list to identify the device file corresponding to your USB stick. Make sure you get this right, as guessing wrong will erase an entire drive you probably wanted to keep. It will be of the format: “/dev/diskN”.
Use dd to write the file to the USB stick:
sudo dd if=bootable.dmg of=/dev/rdiskN bs=4m