Nice! Make sure the battery is installed, then plug in the probes (red in middle spot, black in right spot). Turn the dial to the little speaker/sound symbol as below (I added a little blue arrow to point out out):
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Touch the red and black probes together--you should get a beep from the meter when they're touching. When in that mode, you can test wiring from end to end and if you get a beep, you know the wire has continuity (the meter is sending a small amount of electricity from one probe to the other, and beeps as long as it makes it across)...so the idea is to use the meter to verify that the green/blue wire coming from the MPU at CN8 pin 7 is reaching all 8 of those switches on the chart.
Find those green/blue wires at each of the switches. You should have continuity/beep between the bare ends of the green/blue wire at each of the switches in that column.
For example: when set to continuity/beep test, you should be able to touch either probe from the meter to the exposed/soldered end of a green/blue wire where it connects to any of those 3 yellow target switches, and touch the other probe to the exposed end of the green/blue wire at either of the other 2 yellow target switches. You should get a beep when you do that. Keep one probe on the same wire and move the other probe to the other green/blue wire at the other yellow target switch and make sure it beeps. Then from a yellow target switch to the spinner, or to any of the pop bumper scoring switches.
You should continue getting that beep between any pairing of any of those 8 switches on the column, because the wire "Daisy-chains" between them. That's why you'll see 2 green/blue wires at several of the switches--one's going "to" that switch and another is coming "from" that switch to the next one.
The switch matrix is surprisingly clever yet simple when you understand how it works. You'll get there soon enough! For now--make sure you have continuity between all 8 of the green/blue wires at those switches.
Here's an idea (though I realize I may not be explaining this clearly enough, I figured I'd try anyway)--grab an alligator jumper and clip one end to one of the meter probes. Clip the other end of the jumper to the exposed end of one of the green/blue wires at one of the problem switches. Now you can simply use the other probe to test continuity from the seven other switches with the one that's clipped on to the other probe. If you get a beep from all 7, you can rule out the wiring between the switches. At that point, it'll be a matter of testing between the wiring at the playfield up to the backbox where that same wiring ultimately leads to the MPU.
Ask any questions you have, hopefully this all makes sense!