Below is a copy of an image from the link you gave in your question.
If you enlarge the image, you can read that the source is the file "trusty-desktop-amd64+mac.iso".
This is the WRONG iso to be using. You should be using "trusty-desktop-amd64.iso".
The iso's ending in "+mac" is for users who wish to do a BIOS boot. This includes users whose machines are not Mac's. The regular iso's that do not end in "+mac" are for users who wish to do an EFI boot.
I have found the EFI boots work with 64 bit Macs as early as the 2007 models. If you have a optical (DVD) drive you can boot this way, but the USB flash drive method also works even when Windows USB installer will not. My (20-inch, mid 2007) iMac7,1 can boot Live and/or install current versions of Ubuntu using either a DVD or USB.
Both DVD and USB once booted, have software to verify the media. If found the DVD's verify, but the USB's, created using Ubuntu's instructions, do not. The boots and installs seem to work OK, but the fact the USB flash drives do not verify bothers me. Fortunately, if you create the USB media using UNetbootin, the resulting flash drive verifies.
Evidently, you can also use UNetbootin to create a Live USB version of Ubuntu that has *persistence, but I have never pursued that option.(Persistence allows you to keep your preferences and data even after a reboot.)
I have read may cases where users are happy with BIOS installs of Ubuntu. Problems occur when users what to triple boot of OS X, Ubuntu and Windows. If Windows should be installed in BIOS mode, then the user has to figure out how to get Grub to boot both Ubuntu and Windows. In these cases, booting Windows in BIOS mode and Ubuntu in EFI mode would be a better option. The problem here is that EFI Ubuntu installs do not set up the software to boot on Mac's. You are left to figure that out.
There are may solutions to this problem. Here are a few: First, you can move the files in the EFI partition get the Mac firmware to recognize Ubuntu. Second, you can install and configure rEFInd. Third, since Ubuntu installs a Grub that can recognize hfs+ formatted partitions, you can configure Ubuntu to boot the same way OS X boots. (Actually, the Mac firmware thinks it is booting OS X.)
An explanation of how it implement any of these three method is beyond the scope of the original question.