We would encourage Mac users to download Ubuntu Desktop Edition by
burning a CD. But if you prefer to use a USB stick, please follow the
instructions below.
Note: this procedure requires that you create an .img file from the .iso file you download. It will also change the filesystem that is
on the USB stick to make it bootable, so backup all data before
continuing.
Tip: Drag and drop a file from Finder to Terminal to 'paste' the full path without risking typing errors.
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 the following command while replacing /path/to/downloaded.img with the path where the image file is
located; for example, ./ubuntu.img or ./ubuntu.dmg).
Taken from here:
You can do it really easily with unetbootin which is available for Mac OS - the upside being it doesn't need more than a few clicks on a simple GUI.
(note this is only for newer Intel Mac's, and won't work with older PowerPC machines unfortunately.)
You could try following the instructions on ubuntu.com.
How to create a bootable USB stick on OS X
Useful Links
Additionally, you could try looking at some of these links for help:
Follow these steps:
Download the latest Ubuntu ISO from here and UNetbootin from here.
Insert your USB drive.
Make UNetbootin executable from file properties.
Run it.
Now click in Diskimage. Give the path of the downloaded Ubuntu ISO.
Then select your pen Drive from the list.
Then press OK.
Wait until finish.
After that you are able to boot or install Ubuntu from that pen-drive.
You can use the Linux USB Creator. The Linux USB Creator for Mac has been released. This is the easiest way to create a live linux USB on a mac.
If you use OSX Lion, you can Download this file, and if you use OSX Snow Leopard, you can Download this file
For more information on the Linux USB creator, just visit the Website
The other option is to use UNetbootin.
If your Mac is a G4 or G5 take a look at this.