The installer is looking for a file called Poser.exe and PoserPro has PoserPro.exe so that's why it says it can't find the executable. To get around this, when you run the installer, choose the DAZ|Studio option, it installs the exact same Poser files, it just doesn't look for a Poser.exe, it lets you browse to where you want to install V4.2, which would be your PoserPro folder.