Are you adding an existing sensor to your Pixie timer or moving standalone sensors to a zone?
To move the stand alone sensor to a zone, first you have to go to Settings->Sensors->Edit(upper right corner) and then delete the sensor. Next you will have to go to the home page. There is a drop down menu and please select your timer, then tap the "+" button on the upper right corner, go to Add a Product->WifiDevice-> Plant sensor and then complete the following procedures to add the sensor. You will need to press the button 6 times on the back of the sensor to change its mode to Setup mode (light blinking slowly).
Please let us know if it works.