Honestly, google-surfing this question leads to so many half-answers and multi-part communications that it is both scary and frustrating to try to navigate them.
The question here is simple: What are the clear and complete step-by-step instructions that you used to dual-boot 12.04 on your OSX Lion (entrapped) Apple computer. Did you use rEFIt, rEFIind, a special .iso of 12.04? What?
Obviously, there is a preference for safer, easier, and more reversible methods. I can probably assure that the best answer will get plenty of views.