The LED should light up bright when the digitalWrite(ledPin, HIGH) function is called. If the led is dim, you either have too large a resistor in series with the LED, or you have not declared that the ledPin is an output pin.
Can you post your code, and describe (or show) the hardware setup?
Oh come on, RTT (read the thread). It has been stated clearly that the on board LED (13) is being used and therefore it cannot be made brighter.
Mowcius