Not sure if it is the same but all I did was to buy a 9 pin genesis/megadrive controller extension cable and one 9 pin M-F gender changer (really a tiny extender more than a gender changer).
Connect the extension cable to genny/MD (or even real SMS), connect the gender changer F side to it (to the M on the extension cable), snip pin 7 on the gender changer M side and connect the controller cable to it.
That's it. Basically interrupt pin 7.
What you get is B = 1(Start) and C = 2 (the rest is unused) for a 3 button Genny/MD ctrl
Only that sucks is that you can't use it on MegaED menu' as it wants you to press A, I just connect the pad after I selected and started the game.
All SMS troubled games run fine on MD this way AFAIK, you can use the same contraption to use a Genny controller on a real US SMS, Jap SMS don't seem to mind if you use a Genny controller directly as I believe they do not connect TL (pin 7) anyway so the contraption is superfluous.