Upload post-revision-display.php to the /wp-content/plugins/ directory.
Activate the plugin through the 'Plugins' menu in WordPress.
By default in automatic mode, it should "just work," although you'll want to include CSS for styling the diff/revision comparison and probably also the note at the top indicating when a revision is displayed. See the description tab for sample CSS and instructions on how to use "manual" mode to control where things are displayed.
If upgrading, make sure to check the options page right away. The defaults may not be set right. (If you deactivate/re-activate before visiting the options page, you should see defaults that correspond to the old automatic mode.)
Requires: 2.6 or higher Compatible up to: 3.0.5 Last Updated: 2010-11-19 Downloads: 3,855