I would connect the common to ground and each digit to a digital input with the internal pull ups enabled. The key that is pushed will go LOW. Unpushed keys will read HIGH. This is the most common and accepted way to wire switches.
When working with any circuit like this, use the continuity tester feature on your multimeter and make sure you know the common and what is going on.
Mind you if you take a look at the picture ("View Image"), it is immediately obvious which is the common in this case.