First download a Raspbian RAW image from here: http://www.raspberrypi.org/downloads

Plug-in your SD card into your Mac. Then use diskutil to see the name of the disk device of it:

$ diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *320.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            319.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *4.0 GB     disk1
   1:             Windows_FAT_32                         4.0 GB     disk1s1

In my case it is /dev/disk1.

Unmount it so we can write the image into it:

$ diskutil unmountdisk /dev/disk1
Unmount of all volumes on disk1 was successful

Now copy the image (it will take quite long and you won’t get any feedback until the end so be patient):

$ sudo dd if=2013-09-25-wheezy-raspbian.img of=/dev/disk1 bs=1m
4297+0 records in
4296+0 records out
2199552 bytes transferred in 1.877077 secs (1171796 bytes/sec)