Load this file in your browser (http://yoursite/wp-content/plugins/ozh-admin-drop-down-menu/test.html)
If you see anything but CSS code below, your problem is that your server disables PHP files execution when found in
wp-content directory. It's either done by a .htaccess rule you made, a plugin you uploaded, or sever configuration. In case 1 or 2, you're on your own. In case 3, contact your server admin. In any case: this server limitation is totally useless regarding hacks and stuff. Just thought I'd let you know.