I tried to install Cubase 6 Artist and had the same problem. It thought my OS X 10.10.5 was lower than 10.6.
I began with Terminal as instructed, but after entering the first line (cd ~/Desktop pkgutil --expand “Cubase 7.5.20 Update.pkg” cubase752.unpkg), I got cold feet because this was Cubase 6.mpkg, not Cubase 7.5.20 Update.pkg, I did not have Text editor, and like Johnmarkpainter I did not have a clue what I was actually doing inside Terminal. I exited Terminal hoping that entering the first line of the instruction would do no harm. Turns out it did. When I try to open Cubase 6.mpkg, I get asked which program I want to use, and there is nothing inside Cubase I can choose, because Cubase 6.mpkg is supposed to start the process, and it has been deactivated, obviously. My question is: How can I undo the first line of the instruction in Terminal?
By the way, my OS is not working at the moment and I don’t know when it will, because I tried a different way of managing the problem, by simply changing the OS X number from 10.10 to 10.7. I was a really simple operation, but again I was a fool to mess with Terminal, because now I can’t run several programs including Terminal, because they are based on 10.10.
However, I suppose I should try to get support from Apple rather than Steinberg for this issue. I mention this anyway, because I think you could have made a small installation package to solve the initial problem, instead of making customers use Terminal and risk further complications.