You need to start with an Adruino, your shift reg and its 8 buttons. Then write code to just read the shift reg and out put the result on Serial. Drop all the rest keep it as simple as you can. Post the code and the circuit diagram if you still have problems.
By the way providing that only one button can be pressed at a time then you can have 4 or 8 inputs to a single analog pin. Look in the playground for other way to expand the number of inputs or go to a Arduino mega (simples!)
Mark