Well it has inputs up to 15 for both A and B input which are marked with signs. It also has a lever which selects addition or subtraction. First of all there is a binary decoder. This decodes the decimal input into binary. Then it goes into the adder after going through togglable inverts for B(subtraction uses 2's compliment). If you flick subtraction, it turns on invert b, sets the overflow to zero(you ignore overflow in subtraction) and does CIN. The output of the adder goes into a binary decoder, which then gets decimal output. The output now goes into a 7 seg display code encoder which is finally hooked up to the 7 seg display which displays decimal output.
Flick the levers for A and B input. Then flick or leave the operation choosing lever. If doing subtraction, make sure A is greater than or equal to B. The A and B inputs are labeled so you know which is which. Wait while the machine works out your answer and soon enough will display.
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The 4 bit calculator challenge for Regular should have addition, subtraction, multiplication (4 bit input, 8 bit output for mult), and decimal IO.