Installing Ubuntu 12.04 LTS on Mountain Lion

Solution 1:

This can be achieved using Linux USB Creator For Mac.

Instructions:

• First just select the linux distribution you want to use. You can either select from the list or browse for a Local ISO file to use.
• After choosing Ubuntu you can browse for your USB device.
• The program will then download the ISO, Format the disk, Copy the ISO and Install the Bootloader.
• Your USB Drive is now ready to use. Simply boot up your mac while holding the “alt” key and select the drive.

A YouTube video is also included & can be viewed from the above link to complement.

Please note: This program hasn't been updated since the introduction of OS X 10.8 Mountain Lion.

Solution 2:

You shouldn't directly open the ISO file in OS X: it's not needed. You should simply convert the .img file to an .iso file and copy it to the USB stick using dd, following these instructions.

Then restart your Mac pressing the Option key, and choose the live USB option.