for launch control you need to build the circuit on the board first. i can't remember which one i did, i think i did the ground activated circuit, then hooked the input wire to the 12V side of the cruise control switch. i have not tested launch control yet so i don't know if this works, but it should.
that input wire and circuit is all that is needed for launch control. everything else is done through the settings of megasquirt. the reason why you need the circuit is because the processor can only handle 5V. so the circuit just takes the 12V input and sends 5V to the processor
use the same ignition circuit diagram for the 3.57 board. spark A is the negative side of LED 14, and spark B is the neg side of LED 16
also just a heads up to everyone already running megasquirt. i was reading a post over on turbomiata forum, the older miata's use the same ignition output we do. well one guy improved it by adding transistors so that when you first power on the megasquirt, it won't charge the coils causing an ignition pop if some of you have that problem. it has to do with the bootup lag of the processor. and IIRC if you use the circuit you no longer have to unplug your coils when doing anything that the instructions say to do so, like flashing the firmware. i'm gonna have to find the diagram