You're going to want to double-check this as it is for a carb application, but if you have TB-V ignition it should be pretty close.
The shift cutout switch grounds the ignition for a split-second to aid in shifting gears. There should be a white/black wire and a black wire (common ground) in your loom somewhere, maybe with caps on the connectors or connected together as in the schematic. Those get connected to the two wires coming off the switch on your stbd riser. The diagram has the switch wires as black and white/green, but mine are both black. Doesn't matter, as the connectors only give you one way to hook it up.
Once you find all of your wires and hook it up you can test it with a multimeter or on the hose by activating the switch with a little screwdriver and making sure it's grounding out your ignition.
Hopefully someone smarter than me on the MPI motor will be along to help. Good luck!
Alpha ignition wiring.jpg
"Speed's fine, but accuracy is final."
- Bill Jordan
1998 Donzi 21LXR
1971 Boston Whaler 13