I just re-did this a couple of months back on a 2008 MacBook4,1, also using Yosemite.
There's only one location for the installer, as far as I know. MacPostFactor wants it in the Applications folder. Nowhere else will work. Be sure that you have a good copy. Re-download it if you think you need to do that.
Copy the installer app to the Applications folder. Restart your Mac (might make this work when nothing else seems to help!), and try the MPF setup again.