You can tell whether your heart rate sensor is paired or not by checking the flashing light. If the light is flashing blue and red, the sensor is not paired. If the light is flashing blue, the sensor is paired. If the sensor is not paired, take the following steps to ensure that it is connected over bluetooth.
Turn on year heart rate sensor by holding down the button on the face of the sensor. You should see a light flashing on and off. Make sure the heart rate sensor is charged. Touch the "Settings" icon (blue with a white gear) followed by the tab labeled "Connections."
After touching the "Bluetooth" section, you should see all of the device available for pairing. Select the Scosche Rhythm + heart rate sensor. The device should pair marked automatically. You know it is paired when you see the blue light flashing on and off on the sensor.