WordPress error with this plugin

[saschafoerster]

Notice: wp_register_script was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or login_enqueue_scripts hooks. This notice was triggered by the mailtrain-subscription-widget handle. Please see Debugging in WordPress for more information. (This message was added in version 3.3.0.) in /var/www/html/wordpress/wp-includes/functions.php on line 5663