You need to sync the balance board with the Wii. It's easy to do and takes 10 seconds.
1) Start Wii Fit.
2) When it asks you to push the power button as you're describing, open the SD card slot on the front of the Wii and observe the red button.
3) Now, flip over the balance board. Remove the battery cover on the bottom of it, and press the red button beside the batteries, and the blue light should start to blink.
4) While the blue light on the balance board is still blinking, quickly press the red button on the Wii. If they are both blinking at the same time, they should be syncing to each other.
5) Your Wii Fit Balance Board should now be synced with your Wii! If it has been synced successfully, you will no longer need to sync it, and should now automatically connect when the button is pressed with your foot.