Several people have told you in other threads how to manually install Myth 3 without using the installer and without needing the intermediary patch you can;t find.
You should read those, or scroll back up to the picture of my Myth III install which CLEARLY shows you all the files you need to run Myth III on OS X. As you can see from my explanation (accompanying the picture) I didn't use the installer - in fact my Myth III CD is WINDOWS ONLY CD, but by manually copying the correct files to my Myth III folder and downloading the (displayed) updates I got it to run fine.
Myth 3 1.3.1 PC Update Failure
Re: Myth 3 1.3.1 PC Update Failure
I forgot to emphasize that the significance of the "WINDOWS CD" comment is that I am using a Mac, so I didn't run ANY of the auto installers - the install and ALL the updates were performed manually by creating a folder and dragging the files into it. First the files from the install CD, then the files for the different upgrades until I ended up with the configuration pictured a few posts back.