here I found a video demonstrating that kind of machine, not the same brand and model but very similar. the vid's is French tho : https://youtu.be/vOGtYgu7V7Q

Stick a pen tip into the slot at one of the numbers and scrape it upwards for adding, downwards for substraction.

The display updates with the new sum.

Mine came with more complicated instructions for handling more advanced forms of math, as well, but I immediately forgot those.