Custom Magento admin notifications

Feed.php

<?php
class Meanbee_Notify_Model_Feed extends Mage_AdminNotification_Model_Feed {
    public function getFeedUrl() {
        $url = 
            Mage::getStoreConfigFlag(self::XML_USE_HTTPS_PATH) ? 'https://' : 'http://'
            . 'www.meanbee.com/path/to/your/file.rss';
    }
    
    public function observe() {
        $model  = Mage::getModel('notify/feed');
        $model->checkUpdate();
    }
}

You forgot the return on the getFeedUrl() method.