I have it working for 3.6 too. I had to change to the alt version and make sure I made the first change to pm_showpm and the second change to pm_newpm, but it worked. Also used Mark B's change to get the smilie in and that worked as well.
Also the code you delete in pm_showpm
Code:
<form action="private.php" method="post">
is actually
Code:
<form action="private.php?do=managepm&dowhat=delete&pmid=$pm[pmid]" method="post">
I've attached a version that will work with 3.6.*. I made it 1.0.2. The original author can re-edit and publish as he likes.