The multiplexer is easy.
1: just have a button next to one of 16 display blocks containing "[ADD]0123456789abcdef"
2: and activate that button when {"x,x,x,x" activation->not} equals 0-f
0 == not 1, not 1, not 1, not 1
3 == not 1, not 1, act 1, act 1
The demultiplexer is a bit more difficult...