Do you mean you can't sign into the dyndns web site (as I interpreted your initial post to mean), or that you can't access your controller using your DynDNS hostname?
If the latter...
Click here:
http://www.reeftronics.net/showip.php, and note the IP address shown (that's your current public IP address)
Then, sign into the DynDNS web site, click on My Hosts, and compare the IP adress shown to the one noted above. If they differ, then DynDNS is not being updated.
Do you have DynDNS configured in your router? Are you using the DynDNS software updater?