I'm having the same issue. I clicked on the "tutorial" link you provided, but that link is broken.
I see that the post_templates.php is in the themes section and that it should be in the wp-content\plugins section. All the other plugins have their own folders under plugins, and there isn't one for the post_templates.php file. Is it a simple matter if making a subfolder and copying? Maybe a better question to ask is the tutorial you mentioned still available?