Every domain has to have at least two (2) nameservers.
You can use the Joker.com namerservers
for free, or you may chose your own nameservers. If they already exist, they simply have to be registered with your domain, using "Modify Domain".
If you want to create your own, new nameservers, please have a look at "How to create a nameserver(-record)?" in this FAQ.
Please go to the Servicezone to modify domain namserver properties, or to add or edit nameserver records (URL-forwarding, mail-forwarding, MX-records etc.).
Please Note: The latter only works with Joker.com nameservers, for technical reasons.