How do I direct input/output to correct circuit module. (2 bit calculator circuit)
The last piece you need is a multiplexer, specifically a 1-of-4 multiplexer such as the 74153. Since you have 4 inputs to the decoder you will need 2 chips at 2 multiplexers per chip. Connect the A and B inputs to S0 and S1, each of the C inputs to each of the function output bits in turn, and the Y outputs to the decoder inputs.