In order to setup your domain to use Namecheap's FreeDNS nameservers, you need to change your DNS servers to
  • freedns1.registrar-servers.com
  • freedns2.registrar-servers.com 
  • freedns3.registrar-servers.com
  • freedns4.registrar-servers.com
  • freedns5.registrar-servers.com
at your current registrar. Please contact your domain registrar for information on how to do this in their console.