libfreenect makes controlling the LED on the Kinect very simple, a function called freenect_set_led was created which allows you to specify the device and the LED option.
Set the state of the LED. Uses blocking control message call to update device.
Inputs:
dev – The Kinect on which the LED is
option – LED state to set. (See freenect_led_options enum.)
Returns:
0 – Success
< 0 - Fail
freenect_led_options:
LED_RED
LED_YELLOW
LED_BLINK_GREEN
LED_BLINK_YELLOW
LED_BLINK_RED_YELLOW
LED_OFF
Example Use:
freenect_set_led( f_dev, LED_BLINK_RED_YELLOW );
More information available – here

