For the sake of simplicity, we'll consider that this machine only handles unsigned numbers.
For the addition and subtraction, this does not matter because 2s-complement representation is insensitive to the actual circuit. It's then just a matter of how we interpret the Most Significant Bit, and display the input and output numbers.
Considering that the #DYPLED is not yet ready and I'll be using TIL311s in hexadecimal, it just makes sense to say the numbers are positive.
For division and multiplication, the hardware must know the signedness of the operands. Things can get pretty tricky and division has weird rounding behaviours. These considerations are better left to "advanced" machinery building projects...