Your transformation needs to be something like

select * from xmltable('/x/Root' PASSING jsontoxml('x', '{"Root":{"ElementKey": "Value"}}') COLUMNS ElementKey string) y

of course the json will be replaced with invokeHTTP to get it from a service

How did you use Rest importer wizard for creating the Rest source and View model?

Here are some basics of Teiid. A VDB contains 1 more models

* Model defines schema. This can be physical or virtual

* physical model is representation of the source like database, webservice etc.

* every source, needs a "translator" and "data source"

* A translator converts the SQL into source specific queries. Teiid provides various translators

* A data source is "connection" to the physical source like database, web service etc.

* A view model, does not need a translator or data source, it just a logical schema built using source model's schema

So, in the above "data source" that represents connection to your webservice that returns the JSON is missing. Typically in server you create it with "resource-adapter", Designer can automatically create one for you if followed the steps properly. You can manually create it too. In designer right click on the source model, select "create data source" to create.