Notice the following in the PostgreSQL documentation for 9.2

http://www.postgresql.org/docs/9.2/static/datatype-json.html

The json data type can be used to store JSON (JavaScript Object Notation) data, as specified in RFC 4627. Such data can also be stored as text, but the json data type has the advantage of checking that each stored value is a valid JSON value. There are also related support functions available; see Section 9.15.

There is a difference between storing as TEXT and JSON. JSON fields are not only validated as correct JSON on insert, but you can also index the JSON keys themselves.

When using the Visual Editor to CREATE a table from the Schema Browser you can create a table column with the datatype JSON for PostgreSQL, we just don't allow this datatype in the dropdown for the ER Modeler Table Properties for PostgreSQL.

See attached.