Prop the playfield up at the front so the ball will rest on the monkey magnet. This way you don't have to try and hold the ball on the magnet while initiating the test. Go into tests, Top Lane Magnet Sensor Test. Close the coin door or enable power to the coils with the interlock switch. Place the ball on the magnet and press enter. You should see the Monkey Magnet Sense switch activate when the magnet turns on.

Some people have had to rotate (flip) the magnet sensor board to get it to sense. Some people have had to move it to a different location. I didn't have any luck with either and had to replace the sensor to get it to work.

I went through the same thing. I got a new sensor from JJP, installed it, and it worked fine in the proper location.

The only way I could get my original board to sense was to put it right below the magnet core (bolt). Anywhere else and it wouldn't work. Once I got the replacement, it worked fine mounted back on the backboard.