When you add a domain to the dashboard. You will be given the nameservers to update it too. 

You will need to login to your domain providers control panel. And follow their instructions to update the nameservers.

If you are not sure how to do it. A quick google will usually show you the answers. E.g If your domain provider is GoDaddy, you can just google the question. How do I update my nameservers GoDaddy. And you will find the article you need to follow.

Just remember you only need to add the nameservers. If you are being asked for an IP address or see other DNS records, you are not usiung the correct way to update your nameservers.