Not sure what version you have , but it needs to be set up for a VR sensor to read your crank sensor. Both hardware and software needs to be addressed .
http://www.msextra.com/doc/pdf/html/MS3XV30_Hardware-1.4.pdf/MS3XV30_Hardware-1.4.html
this might get you started . It's a steep learning curve and I have to review it myself. I set mine up years ago it seems, but going strong. You might have to play with the air gap to. I run a hall sensor which may be a bit different , but I think the board is set up the same .
Just noticed this thread , sorry I didn't chime in earlier - trying to restore a 72 in my spare time- lol. Feel free to PM me . I haven't touched my MS for a long time, but it's been dam reliable I guess