I downloaded the latest versions of Post-Plugin Library ( Version 2.5b22 ), and Similar Posts ( Versioni 2.5b22 ).

Both activated without problems. I set a couple of options on the setup’s Options page, and then added the following PHP code to an Affililate Ad widget :

<?php if (function_exists(’similar_posts’)) { similar_posts();} ?>

The widget immediate started displaying related posts for whatever post was being displayed.

Seems fine with WordPress 2.5