On attempt to choose a First Checker when adding a new tld:
Home> External Systems Director> Domain Manager> Domain Zones>
there is a message "No Registrars Supported for that domain zone":
The domain zone is not binded to any WHOIS plugin.
Bind the domain zone to existing WHOIS plugin which supports the zone you are adding or configure a new one.
Home> External Systems Director> Domain Manager> Registrars
The registrar must be based on the Whois pugin. Zone can be added on "Registrar Features" tab.