The Domain Panel allows you to manage your custom domains. It can be accessed from the upper navigation bar, through 'Manage' -> 'Domains' (see figure 1 below). The possible actions are:
Figure 1: The domain panel
First enter the complete hostname in the corresponding domain textbox (e.g.: mydomain.com). You will need to click on Commit for your changes to take effect.
Next you will need to set the following NS records at your registrar (or on your own DNS server):
Certain registrars might require you to add glue records as well. In that case just add the following A records:
A ns1.ddnsmon.com - 188.8.131.52
A ns2.ddnsmon.com - 184.108.40.206
If you're unsure on how to add these records check with your registrar or DNS service provider. Lots of technical articles can also be found on Google.
New domains can take up to 48 hours to propagate through the Internet. The following website can be very useful for detecting problems and monitoring the propagation of the NS records: whatsmydns.net. Make sure you select the proper record type (NS in this case).