Using a Parallax Trackball with an Arduino

Hey folks, I'm new here and trying my best, but feeling a little overwhelmed, and hoping the community can help. I've been tinkering with electronics for about a year and a half now, and this is my first Arduino project.

A client contacted me looking for a cool way to show off his phone at conventions (he's an app developer). I remembered this article Keyless Lock Box - Make:, which uses an optical finger navigation (OFN) as a combination lock to move a servo motor, so I figured I could just modify the enclosure and the code just a little, and I'd be set. Easy!

Well, Parallax doesn't carry the OFN anymore, but since the original article mentioned the trackball (http://www.parallax.com/product/27908), I figured I could just substitute that in, and I'd still be set. Yeah, rookie mistake. The OFN sensor has one output to the Arduino, the trackball has four. So yeah, I'm a little lost.

I was able to find this: http://forum.arduino.cc/index.php/topic,200148.0.html, but I'm not really sure what they mean by displacement, and since there aren't any notes in the code, I'm a bit lost at what some of the code means.

I have read up on what a lot of the code means, but I haven't done any coding since I was doing BASIC programming back on my Atari 130XE. I'm not a dunce, I just new to this and need a little direction on how to get this module to work with the Arduino. Any help you can give is very appreciated.

The trackball produces pulses for UP, LEFT, DOWN, and RIGHT (outputs A, B, C, and D respectively). Each change on each of the four pins means a step in that direction.