If anyone else has this question in the future, the answer from the horses mouth:
Hi Paul,
Adding the button wires is very simple, you should have no trouble if
you can do a little soldering. Look at your mouse board where the
single black GND wire is attached. On either side of the GND wire, you
should see two white silk-screened boxes on the parts side of the board.
They should be labeled something like SW1 & SW3. Those are the left and
right mouse buttons.
Take a look at this pic:
Looking at the back of the board in this orientation, the top 2 holes are the ones that will be connected to your arcade buttons. It doesn't matter which hole is
connected to the GND or NO tab on your microswitch, it just needs to
complete the circuit when the button is depressed.
This is where the soldering comes in; you need to solder a wire from
each of the two holes on the mouse board so you can connect a
pushbutton.
I hope that helps!
Kelsey
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Thanks again Kelsey!!!