Looks like you have to enable JSON Data Source as a type. To do that, you will need to edit the applicationContext-remote-services.xml file on the server. The "To enable file data sources in the UI" instructions on the Admin Guide page that you linked to in your question tells you how to edit the file to enable the JSON Data Source type. Basically, under <util:set id="customDataSourcesToHide"> in that file, you will need to edit the jsonDataSource value to make it look like "<!-- value>jsonDataSource</value -->" (without the quotation marks). Hope this helps.
