I am having the same issues getting the white listing to work.
If I use the same custom list I created and black list it, the browser will shows the black list that blocked it. However if I add it to the white list, it denies the URL stating that it falls under one of the default categories.
It acts as if the list did not exist. I have tried modifying the firewall through NSM as well as the web gui with the same results.
Current Firmware Version: 6.0.0r5a.0