The diagnostic LEDs are located to in a column next to the memory slot. Please see the image below.
LED 1 : Trickle power from the power supply to main logic board.
LED 2: Main logic board has detected trickle power from the power supply when the unit is powered on.
LED 3: Computer and video card are communicating with each other.
LED 4: Computer and LCD panel are communicating with each other.