1) you've not created an authhash for the stream ->
http://wiki.winamp.com/wiki/SHOUTcas...ash_Management
2) you've not opened the ports correctly which prevents the DNAS from being correctly seen on the internet ->
http://wiki.winamp.com/wiki/SHOUTcas...ll.2FISP_cache. (though you mention you have which is why i've listed it second if only to re-check if #1 is not the case).
additionally, you can check the DNAS's logs for any
error messages being reported especially being prefixed with
[YP] or
[YP2].