Here are the steps for a brand new SD card, assuming that diskutil list returns /dev/disk4 as the location of the SD card (be very careful and make sure you are using the correct device before formatting!).

diskutil list

sudo diskutil eraseDisk FAT32 UBUNTU MBRFormat /dev/disk4

sudo diskutil unmount /dev/disk4s1

sudo dd bs=1m if=/Users/norris/iot/rpi/2015-11-13-ubuntu-trusty.img of=/dev/disk4