Note: this procedure requires an .img file that you will be required to create from the .iso file you download.
TIP: Drag and Drop a file from Finder to Terminal to 'paste' the full path without typing and risking type errors.
 ▪ Download the desired file
 ▪ Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight)
 ▪ Convert the .iso file to .img using the convert option of hdiutil (e.g., hdiutil convert -format UDRW -o ~/path/to/target.img ~/path/to/ubuntu.iso)
 ▪ Note: OS X tends to put the .dmg ending on the output file automatically.
 ▪ Run diskutil list to get the current list of devices
 ▪ Insert your flash media
 ▪ Run diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2)
 ▪ Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2)
 ▪ Execute sudo dd if=/path/to/downloaded.img of=/dev/rdiskN bs=1m (replace /path/to/downloaded.img with the path where the image file is located; for example, ./ubuntu.img or ./ubuntu.dmg).
 ▪ Using /dev/rdisk instead of /dev/disk may be faster.
 ▪ If you see the error dd: Invalid number '1m', you are using GNU dd. Use the same command but replace bs=1m with bs=1M.
 ▪ If you see the error dd:/dev/diskN: Resource busy, make sure the disk is not in use. Start the 'Disk Utility.app' and unmount (don't eject) the drive.
 ▪ Run diskutil eject /dev/diskN and remove your flash media when the command completes
 ▪ Restart your Mac and press alt while the Mac is restarting to choose the USB-Stick