I tried to install Ubuntu by following this tutorial, however finding out that when I tried to boot Ubuntu it gave me the message operating system missing.
Does anyone has an idea what I should do now, or maybe help in providing another solution?
What I did was I installed Windows 7 via BootCamp and then installed Ubuntu via Wubi.
This was a lot simpler than the tutorial that you linked to but it's also more expensive (i.e. you have to purchase a copy of Windows) and you didn't indicate whether that was even desirable.