Identify the disk (not partition) of your SD card e.g. disk1 (not disk1s1):
diskutil unmountDisk /dev/<disk# from diskutil>
e.g. diskutil unmountDisk /dev/disk1
its easiest to change into the folder containing the image first (use cd <folder name>), and rename the image file to something simple (I always call mine raspberian.img)
sudo dd bs=1m if=raspbian.img of=/dev/DISK
e.g. sudo dd bs=1m if=2014-01-07-wheezy-raspbian.img of=/dev/disk1
And then wait! It looks like nothing is happening, but something is! It is writing the image to the SD card. Depending on the speed of the card this can take several minutes.