One does not simply unmount a USB flash device with the OS X "eject" feature. Instead you'll need to run an OS X unmount command to put the device in a state ready for DD.

Firstly find our the disk identifier of your flash memory.

diskutil list

My disk identifier was /dev/disk1 so lets unmount /dev/disk1 with OS X's disktil command and finally run DD.

diskutil unmountDisk /dev/disk1
sudo dd if=some.iso of=/dev/disk1 bs=8192

Obviously replace some.iso with the name/path of your image file.

Please Note: DD returns no feedback during the copying process and an entire ISO can take a few minutes so just sit tight and be patient!

Finally eject your disk.

diskutil unmountDisk /dev/disk1