This is my code to retrieve feedkey from wordpress db for logged user:
<a href="
<?php
global $current_user;
$feedvalue = $wpdb->get_var("SELECT meta_value FROM $wpdb->usermeta WHERE user_id='$current_user->ID' AND meta_key='feed_key'");
echo (get_bloginfo('url').("?feed=rss2&feedkey=").($feedvalue));?>">Post RSS Feed</a>
If it can be helpful
