the power board also creates the negative voltage needed for LCD contrast. without it, you have no contrast in the LCD...
you can hook up a power LED anywhere on the board, you just need to find ground and the regulated 5V (if you have a multimeter it helps). otherwise, there are some pics floating around which have 5V places and GND marked...
then, you need a resistor to limit the current based on the LED type you are using.
google "LED resistor calculator" and you can find some nice sites where you just enter in the info for your LED, and it will tell you which resistor to use. 5V will burn out an LED otherwise, unless its the kind with internally installed resistors rated for that voltage (more common rated for 12V)
hope that helps!