I believe this plugin will not work as written on a blog that's set up anywhere but at the root of the site, or at least that's my experience.
In commentsvote_js_header() you're neglecting to global $wp_object_cache, so when you set the path the siteurl portion of the path is empty, so it tries to locate the wp-content folder at the root of the site. I was not getting the styles or images, and comment vote submissions were returning 404s since the commentsvote_ajax.php file was not at the expected location. Adding $wp_object_cache to the global on line 44 resolved the issue.
I'd leave a comment on the plugin page, but comments seem to be closed.