Instructions how to make a Raspbian bootable SD card for Raspberry Pi.
Download the most recent version from Raspberry Pi, via their general page, or direct link.
Insert the SD Card into your laptop. It must be a single partition volume, formatted FAT32.
Run mount to see where the SD card is.
You can see here that
/Volumes/NO NAME is on
Unmount the volume so that we can freely write on it:
diskutil unmount /dev/disk2s1
sudo dd bs=1m if=~/2015-05-05-raspbian-wheezy.img of=/dev/disk2
dd is not the most talkative command, so it will just go silent while it works. Use Ctrl + T to get it to give a status update.
Once done, use
diskutil eject /dev/disk2 to eject the SD card.