hi
this information is out there, but it's written for pics, basic stamps and other processors. It should be easy to port it over to the Arduino. Just Google some of the following to get started:
keypad basic stamp,
orkeypad pic,
orkeypad picbasic,
orkeypad atmega8,
or the ever-popular Arduino keypad!
Within the results for these searches you will find dozens of pages describing how to do it for other platforms. Once you get started, people in here are generally happy to help you fill in the details.. but you have to initiate the research yourself
D