First, verify the path to your SD card. You can do this by running the following command from terminal:

diskutil list

The output shows a list of disks currently mounted on the system. Here's the relevant line from my output:

/dev/disk3
#:                       TYPE NAME                    SIZE       IDENTIFIER
0:     FDisk_partition_scheme                        *8.0 GB     disk3
1:                 DOS_FAT_32 RPISDCARD               8.0 GB     disk3s1

In this case I can verify /dev/disk3 is my SD card because the TYPE, NAME and SIZE values are correct.

If you have an existing partition on the disk you may need to unmount it, otherwise you'll get a "Resource busy" error message when you try to write the image.

diskutil unmount /dev/disk3s1

Now to write the image file to the disk. Note the 'r' added to rdisk3 which drastically improves write performance by telling dd to operate in raw disk mode:

sudo dd if=RetroPieImage_ver2.3.img of=/dev/rdisk3 bs=1m

Depending on the size of your SDcard this may take a while. You can press CTRL+T to see the current status of dd. As an example for my Samsung 8GB SD card the write speed was 12MB/s the command took 11mins to complete.

Try this: ApplePi-Baker

It's free, writes IMG files to SD-Card, can prepare a NOOBS card and can make a backup to IMG of your SD-Card.

You could also try: dd Utility

Features:

• Write IMG files to memory cards and hard drives.
• Backup and Restore IMG files to memory cards and hard drives.
• Install and Restore compressed disk image files on the fly. Supported file formats: IMG, Zip, GZip and XZ.
• Backup and compress disk image files on the fly in ZIP format to significantly reduce the file size of backups.
• Ideal for flashing IMG files to SD Cards for use with Raspberry Pi, Arduino, BeagleBoard and other ARM boards.
• Mac Retina displays supported.

I made a script to burn .img or .iso files to SD card or USB.

Github > burn.sh

Yes the simple answer is to just dd it, but there are some safety precautions you may want to enforce by wrapping your dd in a script;

#!/bin/bash

#
# copy_img_to_sd.sh
#

ME=$( id | grep root | wc -l | perl -p -e 's/[^0-9]+//g');
if [ "$ME" != "1" ] ;then
    echo "must be root"
    exit 1;
fi
IMG=$1
if [ ! -f $IMG ] ;then
    echo "can not find $IMG";
    exit 2;
fi 
DISK=$(ls -l /dev/disk? | wc -l |perl -p -e 's/[^0-9]//g')
if [ $DISK -lt 3 ] ; then
    echo "can not find sdcard";
    ls -l /dev/disk?
    exit 2;
fi
DISK=$(ls -1 /dev/disk? | tail -1);
R_DISK=$(ls -1 /dev/rdisk? | tail -1);
echo "we are about to do this:"
echo $(diskutil information $DISK | grep Total)
ls -1 /dev/disk?s* | grep "$DISK" | perl -p -e 's/^(.*)$/diskutil unmount $1;/g'
echo dd bs=1m if=$IMG of=$R_DISK
echo sync
echo diskutil eject $R_DISK
echo "Press [enter] to continue or [Ctrl]+[C] to cancel";
read YNM;
ls -1 /dev/disk?s* | grep "$DISK" | perl -p -e 's/^(.*)$/diskutil unmount $1;/g' | bash 2>/dev/null
dd bs=1m if=$IMG of=$R_DISK
sync
diskutil eject $R_DISK

Accidentally writing an image to your internal drive will require a fresh OS X install to correct. Backup and keep an install.log so if it ever happens to you you can laugh it off.

Found a really good link: http://www.tweaking4all.com/hardware/raspberry-pi/install-img-to-sd-card/#macosx for installing file.img on SD card, very detailed steps!

This app may be helpfull for you: http://alltheware.wordpress.com/2012/12/11/easiest-way-sd-card-setup/

Welcome ;)

Use df to find the device path, in this case /dev/disk2.

$ df -h

Filesystem      Size   Used  Avail Capacity   iused    ifree %iused  Mounted on
/dev/disk1     465Gi  414Gi   51Gi    90% 108573777 13263821   89%   /
devfs          214Ki  214Ki    0Bi   100%       741        0  100%   /dev
map -hosts       0Bi    0Bi    0Bi   100%         0        0  100%   /net
map auto_home    0Bi    0Bi    0Bi   100%         0        0  100%   /home
/dev/disk2s1   3.7Gi  2.3Mi  3.7Gi     1%         0        0  100%   /Volumes/UNTITLED