When I go
Top > Service Director > Domain Manager > Domains > domain.tld, press Register/Transfer, then Transfer an existing domain, select a domain registration hosting plan and press Next I receive the error
Domain 'domain.tld' cannot be registered. TLD is not activated for the selected hosting plan.
.tld is not activated in the hosting plan and/or in the domain registration plug-in.
Activate tld in the hosting plan as described in Activating and Configuring TLDs Per Hosting Plan
Configure domain transfers for tld as it is described in Setting Registrar Prices