Had no problem installing Poser Pro 2012 on my MAC.

Created the new content folder with Poser Pro's 2012 runtime in my application folder, but not inside Poser Pro's 2012 program folder. Then created a new library in Poser Pro that links to my old  Poser 8 Runtime.
I have to say that loading stuff from the linked Runtime is a bit slower. 

Are you saying you are trying to merge old and new Runtimes during install??