Guide to gyro and accelerometer with Arduino including Kalman filtering

Hi chaps. I've been playing around with this board and the code and I must say, thanks for this. Its definitely been a big help.

Im porting this to C++ and Cinder for use in an experiment. I need to get all 3 axes represented though. Is there any particular reason that the Z Axis is not represented?

I notice that altering the Y rotation also alters the Z rotation on the Gyros but not on the Accelerometers. I believe I need to compute the Z somehow but I'm really not too sure on the math.