First of all download the iso from here I've downloaded the 64 bit version.
# show your devices and detect which one is the USB you want to use, in my case /dev/disk2
$ diskutil list
# clean it
$ diskutil eraseDisk FAT32 kali /dev/disk2
# unmount it
$ diskutil unmoundDisk /dev/disk2
# write the image on it
$ dd if=<your-image.iso> of=/dev/disk2 bs=1M
# unmount it, and you are ready
· diskutil unmountDisk /dev/disk2