After you login you will see this form:

Just fill in the name of the domain and click Add.

Tip: You can add more than one domain at once by entering a comma separated list of domain names.

You cannot rename a domain name. Just delete it and create a new one.

You can edit the records in a domain by clicking on its name or on the pencil icon.

You can delete a domain by clicking the red icon next to it. GeoScaling will allow you to shoot yourself in the foot, as it doesn't ask if you really want to delete it.