You can try doing this, but be careful not to point out the wrong output :
if= "your iso file" of= "your usb stick", make sure of the device name before doing it, this can cause GREAT DAMAGE to your partitions and datas
dd if="your iso file" of="your usb stick" bs=512k
source : http://docs.kali.org/installation/kali-linux-live-usb-install