How to create a bootable USB stick on Mac OS X

1Download Ubuntu Desktop
2Open the Terminal (in /Applications/Utilities/ or query Terminal in Spotlight).
3Convert 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.
4Run diskutil list to get the current list of devices.
5Insert your flash media.
6Run diskutil list again and determine the device node assigned to your flash media (e.g. /dev/disk2).
7Run diskutil unmountDisk /dev/diskN (replace N with the disk number from the last command; in the previous example, N would be 2).
8Execute 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.imgor ./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

9Run diskutil eject /dev/diskN and remove your flash media when the command completes.
10Restart your Mac and press alt/option key while the Mac is restarting to choose the USB stick.

From: http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-mac-osx