1. Insert your flash drive
2. Open Terminal and run :
diskutil list
3. Note the device node assigned to your flash media
(e.g. /dev/disk1s1)
4. Run :
diskutil unmountDisk /dev/disk1s1
5. Then execute :
sudo dd if=/path/to/downloaded.img of=/dev/disk1s1 bs=1m
6. Finally Run :
diskutil eject /dev/disk1s1
7. Remove your flash drive