I recently downloaded and tried to install the latest beta release for Poser 5, and received the same error message--that it couldn't find Poser 5. I uninstalled and reinstalled Poser 5, and the patch installed just fine. I had reinstalled Poser 4 and Pro Pack after Poser 5 which was installed when I first got it. Rarely use Poser 5 because I don't have time to relearn it. I would suggest reinstalling it.

Poser 2, 3, 4, 5, 6, 7, 8, 9, 10, Pro 2014, 11, 11 Pro