I did this successfully last summer. There was a gotcha in the process, and I wish that I wrote it down!
It may have been that OS-X mounted the card after it was formatted, and it needed to be un-mounted before hitting it with the dd command. Here's my root user's (sudo -i) command line buffer history from around that time:
diskutil unmount
diskutil unmount /Volumes/RECOVERY
diskutil list
dd if=mhrpi-v5.0.img of=/dev/rdisk3 bs=1m
diskutil list
Note that Unix variants use device names starting with "r" to denote the raw device, which dd needs to work its magic. Linux does not use this convention. Oh, and obviously make sure that you pick the correct disk, since it is assigned randomly and probably won't be rdisk3 on your Mac.
As an additional reference, you might want to check the Raspberry Pi docs for how to make a bootable Noobs SD card on a Mac for other hints. I didn't need to use their third-party flash formatter program, IIRC. The built-in Mac stuff worked fine.....