This is a quick and dirty way of addressing the pins on a 12 key common terminal keypad with an Arduino type micro-controller dev board. The keypad is branded Seeedstudios and I got it from the RadioShack Electronics Component Pack 2. The "Arduino" today will be an UNO Rev3 clone.
Step 1: Program the Controller
First, take your bare Arduino board without anything hooked up. I strongly recommend programming the board before hooking anything up to it. You don't want to accidentally short an output pin set to high to ground!
Open the code in Arduino's software and compile and upload it.(Or do whatever you do to program your Arduino.)
Step 2: Connect the Pins
The easiest way to do this would be with a solder-less breadboard and jumper wires. Jump each pin directly to the headers on the Uno.
When facing the buttons of the keypad, the leftmost pin will be the common terminal("pin 1" see image). Wire this pin to ground. Then wire the remaining pins from left to right(pins 2-13) to Arduino pins 2-13 respectively.
Now there's nothing left to do but reconnect the Arduino and try it out.