If you are using the data path drawn by Eric Mercer, tie the ADSU8's ADD input to Vdd, run your Add/Sub line into the CIN and the SELECT line(s) for your inverter MUX (keeping in mind that you need to invert the input buss for a subtract, and that Add/Sub is low for add, high for subtract) - and all will be peachy. Basically take advantage of the two's complement system and ditch the "sub" functionality altogether.
I believe the problem with the IamAnon's idea is the amount of double inverts going on in his suggested logic. Why is he using the "sub" function at all? It's hella confusing!