Brian, we have some basic information about the extension on our Help Center. I looked up the digi template, and the available services looks to be quite similar to that of streams. Were you able to bind any properties? If so, can you try setting the property values to logged and then running the QueryPropertyHistory service?
Also it should be noted that the Digi Extension is to connect to the Digi Device Cloud, not directly to the devices. The digi devices are not setup to allow a Thingworx EMS to run on their OS (though you could do Rest API calls if you setup the device to do this). Not sure if you were connecting to the Digi Cloud or directly to the device.