There are a few reasons why your sensors may not show up in your available devices when you tap the "add devices" button in the workout player device management screen.
One Connection at a Time
Bluetooth connections are mostly monogamous. Each sensor can only make one connection to an app or device. If you have other apps or devices that connect to the sensor you're trying to connect, please power them off or force quit the app to release the Bluetooth connection. Turning off Bluetooth on other devices will also release the connection so that it is available.
Bluetooth devices must be within close proximity of each other to connect. Make sure your sensors are within 6 feet of your app. If they are within 6 feet and still do not connect, consider replacing the battery to strengthen a weak Bluetooth signal, or unplug the device and plug it back it after 30 seconds to refresh the signal.
Connect through the SYSTM app only
Make sure your sensors aren't directly connected to your device (phone, tablet or computer) through the device's Bluetooth connection. If they are, remove those connections and then connect the sensors through the app only.
Reboot your device. This often resolves any pairing issues.
Move away from any sources of signal interference such as Wifi routers, boosters and extenders. Other sources of interference are microwaves, high-powered fans, large electric appliances, and other Bluetooth signal sources such as Bluetooth speakers. Power off what you can't move away from.
Check to make sure that the sensors you are trying to connect are compatible with the device you're trying to connect to.
Check to see if your sensor has any available firmware updates. Be sure to close off the sensor's parent app once this is completed in order to release the Bluetooth connection for use in the SYSTM app.