I had tried to configure Data Viewer/Setting/Geiger Counter World Map before registering on the Map, and when it asked for Geiger Counter ID, I didn't know what that was and guessed it was the device's serial number, which I entered (in hex!). In hindsight I should have realized that Data Viewer could have retrieved the serial number itself if that's what it had needed.
After I changed it to the ID assigned by the map, it started working.
I found the Tutorial on the Map page after I got it working.