Some devices require connecting a cloud account for access.
Visit “Accounts and Hubs” in the app’s settings menu, or run discovery from the Things tab and add the account manually.
If your device is connected to your local wi-fi network (e.g. a Belkin Wemo or TP-Link product) and isn’t discovered, try re-running discovery. Some devices can become less responsive to discovery requests when they are idle for extended periods of time.