There is no "ON" button on the device. Sensors are generally delivered in what we call "Sleep Mode." Before putting a sensor in the soil, you can activate it (or "wake it up") by grabbing it by the pins at the bottom, turning it upside-down for 2 seconds and then turning it right-side-up again. An accelerometer in the sensor will tell it to start the connection process with the nearest gateway within range.
Even if the sensor is not woken up, it will recognize soil contact and start the connection process. In this case, however, the command to begin the connection process may take a bit longer.