Need assistance with changing DNS server

I have migrated from WEBcs to Host gator. Host gator has assisted me with migrating the emails to the new server, but now they want me to change DNS pointing or MX records so the e-mails can be enabled at Host Gator. I checked the website where I had registered the domain, but it shows “We can't display your DNS information because your nameservers aren't managed by us” so I need you to check where to change my DNS records. I have two website and need to fix both.