This feature is officially disabled by default in the latest firmware (20161201) due to numerous reports of LEDs not adhering to the dimming commands. I even discovered a panel at home with this problem. The feature appears to be half-baked by the LED manufacturer, and is unsupported by the LED library I use.
That said, if you want to try this totally unsupported feature, you can add the following line to your WIFI.INI file on your microSD card inside 00system/wifi. Just make sure it appears somewhere beneath [network].
apabright = true