Download your .img file archive.
Extract it the .img to your desktop.
Insert your blank SD card.
Identify it from terminal (diskutil list) or Disk Utility (select the disk, then click File >> Get Info and read off the Disk Identifier).
Unmount (not eject) any volumes on the SD card, again with the gui or from terminal (diskutil unmount diskxsn where x is Disk Identifer and n is partition ID)
Finally write the image to the unmounted disk using terminal:
sudo dd if=RPi-TC_ARMv6j_R5.img of=/dev/disk4 bs=16k
You can see how far along the dd write process is at any point by pressing CTRL+T in the terminal window; the equivalent of passing SIGUSR1 or SIGINFO to dd: