Are you referring to the Maintenance/Dynamic DNS options?
Reviewing the Dynamic DNS options under Maintenance seems to only have preset options for the various services listed and I don't see an option to enter in a different or custom DNS service information here.
User name should be the account User and Password that was created and used to log into that DNS service.
If this is a new router, phone contact should be free initially I would think.
I would check on free phone support, if not, try to email DLink support and see if you can get help there.