The plugin is activated, but it says that the plugin is not tested with my version of WordPress (version 4). I followed these instructions from the Installation Instructions tab:
B) If you want to show the thumbs after all your content (posts, pages, custom post types) paste this snippet at the end of your functions.php file of your theme:
function thumbs_rating_print($content)
{
return $content.thumbs_rating_getlink();
}
add_filter('the_content', 'thumbs_rating_print');
I get a 500 error saying that the function thumbs_rating_getlink() is not defined. I did a search in the thumbsup plugin directory for the function and found nothing, so I’m inclined to believe that.