To input the card codes go to Grandpa's Card Shop and on the upper right table there will be a laptop(computer). Press the A button when you are standing in front of it. You'll be asked if you want to input a card password by paying 1,000 Zennies. Press on the upper word if you want to input the code or on the bottom word if you don't want to input the code. Be carefull with the codes, because you don't want to waste 1,000 Zennies on a single code.