WordPress.org

Does this work with ugly permalinks? (?p=123)

Of course! :)

I've installed and enabled the plugin but nothing changed! What's wrong?

Unfortunately, the dates are not converted automatically in WordPress versions prior to 2.x. Don't worry though, the plugin is still working but you have to replace some function calls on your own. In most of the cases, you should replace the_time(); with clickable_time(); and the_date(); with clickable_date();. Just have a look at your themes files index.php, single.php, category.php and archive.php and you will find what are you looking for!