When you first start out in the Kid Power App, there is an in-app tutorial that walks you through linking your band.
After you've completed a training exercise of applying 750 free steps towards the mission progress, you'll be guided through the tracker linking process as you keep pressing the Move button in the bottom of the screen. You should be able to link your band from the instructions on the screen.
You can also link your band by tapping on your avatar in the bottom left of the screen to load your Profile. From here, tap on the edit pencil on your avatar to see the option to link your tracker.