The EmailTemplateManager utility assumes that the CentraSite registry/repository is running at http://localhost:53307/CentraSite/CentraSite (i.e., it assumes that it is installed on the same machine as the EmailTemplateManager utility). If you installed the registry/repository component on a different machine than the CentraSite Application Server Tier (CAST) or if you configured the registry/repository to run on a different port than 53307, you will need to use the -dburl parameter to specify the address of the registry/repository when you run your script.