A usage example of the Display Actions/Get Geolocation plugin is available in http://www.tersus.com/#Id=3685.
The location is retrieved from the browser which in turn retrieves it from the device per the permissions granted in the device settings (GPS, network-based location, etc.).
You can safely ignore the warning you mention, and to do so permanently you can click 'Ignore selected warnings' button in the validation view toolbar. You can also remove the data element, as it is there solely as a hint regarding the plugins output format, and removing it will have no effect on functionality.
For best results, use the Firefox browser..