The ESP8266 devices have to conform to certain API protocols for the app to work.
Goto Settings for this app and enter the Switch password, Name you want to appear in the label and its IP address in this format (192.168.2.45 or 192.168.2.45:5001 or 175.56.34.89:5002)
You can have 7 devices programmed, 5 on the first page and 2 on the second