Visit the servicezone at the main menu of Joker.com .
Go to "Domain-Modification" and select the domain for which you want to use the Joker.com nameservice.
in the nameserver fields and delete all other nameserver entries.
Submit the modification and visit "Nameservice and Webforwarding" in our service zone.
You will notice the appearance of your previously modified domain. Now
you can enter all nameserver-zone details for your domain.
Please consider reading the documentation you find on these pages.