You need to run the IoT Inspector Desktop App before you can inspect your home network's IoT devices.
If you have already downloaded IoT Inspector, simply run it. Otherwise, download it here.
This page will automatically refresh once the IoT Inspector Desktop App is running in the background.
If this page does not automatically refresh after 30 seconds, make sure to open https://inspector.cs.princeton.edu/ in a separate Firefox or Google Chrome window (or any Chromium-based browser).