To begin, the guide you reference is badly outdated; its page history indicates that its last edit was in 2013. It relies heavily on rEFIt, which has been abandonware since 2010. I maintain rEFInd, which is a currently-supported fork of rEFIt that is much better for dual-booting with Ubuntu.
Also, you haven't said what Mac you're using (especially its age and, most importantly, EFI bit depth), what bit depth of Ubuntu you're trying to install, or what version of OS X you're running. These details are critically important, because an accurate answer for one set of these details will be badly misleading for another one -- or the answer will have to be ridiculously long to cover all possibilities.
To get you started, though, I recommend you read:
- The rEFInd documentation on SIP -- This is a feature of OS X 10.11 that interferes with the installation of both rEFIt and rEFInd. Of course, this is irrelevant if you're using an earlier version of OS X.
- Instructions on installing rEFInd under OS X -- You may be able to combine this with the page you used to get mostly working.
- My page on the CSM -- Many older Ubuntu-on-Mac installation instructions rely on the Mac EFI's Compatibility Support Module (CSM), installing Ubuntu in BIOS/CSM/legacy mode. This is a Bad Way to do it these days, and my CSM page describes why. That page is mostly written with UEFI-based PCs in mind, but it's still mostly applicable to Macs. Understanding these issues will help you get things working correctly.
Note that it may be best to install Ubuntu on the computer before installing rEFInd in OS X. The reason is that this order will cause the refind-install
script to set up a filesystem driver that will enable rEFInd to launch the Linux kernel directly and in EFI mode, rather than relying on either BIOS-mode or EFI-mode GRUB to get Ubuntu to boot.