Here is the schematic:
(http://i272.photobucket.com/albums/jj190/simonbelmont2/Megadrive-GenesisSwitchlessMod.jpg)
About the resistor: the minimum is 200Ω and the maximum is 1kΩ.
About the reset: pin 18 at the CPU 68000.
If you want to have a brighter led use less Ω.
In my example I use 300Ω R.
Credits go to Calpis from assemblergames (I've inspired from his schematic).
If you have questions, don't hesitate to e-mail me.