The files are available at http://code.google.com/p/pnotepad/downloads/list.
Be sure to get the version that matches the Python version you use. Using a wrong version will cause problems. If you have multiple versions of Python installed, running “python” in dos prompt / powershell window will run the default version used by Windows. Observe what version gets started; that is the version you'll need PyPN for.
Extract the files into the PN install directory; typically this is “C:\Program Files\Programmer's Notepad\”.
After this, run "pn.exe --findexts". That will tell PN to search for extensions and register them.
Note: this will have the effect of adding <extension path="pypn.dll"></extension> to config.xml
If everything went as it should, you can then open the Scripts window from the menu (View → Windows → Scripts or just press Alt+F10). Try doubleclicking on scripts and see what happens.