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Set up the plugin as per usual and it'll appear as a widget to be configured, where you can configure the amount of posts displayed.

There is one additional change. In your theme, open the single.php file and add <?php if (function_exists('ptw_countview')) ptw_countview(); ?> inside the loop. If you want your post views to count when seen elsewhere (not just on their own), insert that code everywhere else too.