You can only "point" a domain to 1&1 name servers if the domain is currently registered with another company. Pointing a domain to the 1&1 name servers is an alternative to the Transfer of a Domain to 1&1.
If you do not see the option to Point your domain to 1&1 name servers please scroll down to the Troubleshooting section at the bottom of this article.
If unsure what to choose, Use the 1&1 mail servers. Then click the Next button to proceed.
Click the Order button when you have taken note of the name servers that should be used for your domain. Please note that there is no charge for DNS transfers.
The Status will read DNS settings incomplete with registrar until you contact the domain's registrar and supply them with the DNS servers provided to you in the last step. After the domain's DNS has been updated, it should only take 24-48 hours to update at which point the domain's status should change to Ready.
1.) The domain is already registered with 1&1 and therefore must go through a Transfer of Registrar process to move the domain into another account. There is no way to perform a DNS transfer on a 1&1 registered domain as the domain will already be using the 1&1 DNS. This is why you must follow the Transfer of Registrar process to transfer the domain.
2.) The domain is registered elsewhere but already pointing to 1&1 DNS. The domain is likely set up in another account already as an external domain (DNS transferred domain) and therefore can not be added to a second account as a DNS transfer. The account holding the domain must "delete" this external domain before you will be able to add this domain to your package as an external domain.