Right from the WIKI (and LED's are current based. Most only require 1.5v to 3v, it's the resistor that allows them to work on 12v)
Optional LEDs
The LEDs are optional, but if you choose to use the LEDs you MUST install the resistor.
'Parts'
2 x Surface Mount LEDs
1 x 1K ohm Surface Mount Resistor
Mount the LEDs in different directions (polarity) so that one of the LEDs will light up regardless of the board's polarity. You can use a single LED if you are certain of the polarity.
PCB HEATBED DIAGRAM r.jpg