WORKAROUND:
Create a new User Data Type called "Hstore". Make sure to un-check the box "Generate" in the general tab of the properties window since it's not declared the same way as a default data types are. Once completed, Hstore will become available as a data type for other attributes.
An important tip if planning to generate DDL scripts for the model with the Hstore data type; make sure the following command is included in SQL code before any entities are created:
CREATE EXTENSION hstore;
Or enter it into Before Script tab section, which will add it for you when DDL is generated.
