If it's like the Stages G2 you don't pair cadence separately, if it finds and pairs the power meter you will also get cadence. Only thing is, it won't give you cadence by just back pedaling or lifting the back wheel and pedal indoors. You need to ride properly and apply power to the crank arm to get cadence.
With either version of the Stages power meter, you might need to move the 820 close to the Stages meter to get it paired initially. After it is paired it will usually find the Stages meter even when starting the 820 on the handlebar or stem. The cadence is included with the pairing of the Stages power meter so if you are getting a power read out you should also get a cadence read out. No additional step is needed.
I have paired my Stages V1 without any problems. The 820 shows power and cadence but it's true it only shows cadence when power is also detected (meaning during a bike ride). If pairing does not work you could try inputting the sensor ID manually.
