The multiplexer chip used in this CPU is a 16:8 mux - this means it can select between two 8-bit inputs.
Don't worry, this is an extremely simple chip to construct. To start with, you must build from logic gates a 2:1 mux which can switch between two individual bits. Then you can use 8 of these 2:1 muxes to build this chip.
Once you are finished, you can return to the CPU.