You need to specify your domain name in the username field.

Please try using the following setup:

DDNS Service: Custom
DYNDNS Server: dynamicdns.park-your-domain.com
User Name: yourdomain.com
Password: your dynamic DNS password this is not your login password
Host Name your subdomain
URL: http://dynamicdns.park-your-domain.com/update?host=@&domain=yourdomain.com&password=12345

PLESE NOTE: you need to change the "yourdomain" and password in the url 



Tested and works on a linksys Firmware: DD-WRT v24-sp2 (03/25/13) mega