[DrunkenBrit]

Maybe something along the lines of:

diffInZ = 3.0 # Value to add/subtract to Z position of selected ball

def onMouseWheel(dir):

    global ball # Same ball object selected from previous mouse move callback

        # Check 'ball' is valid/one selected from previous callback
               ballPos = ball.getPosition()
               addZ = -3.0            
        
               # If wheel is moving forward, make positive, else leave as negative
               if dir > 0:
                 addZ = 3.0
            
            ball.setPosition( ballPos[0], ballPos[1], ballPos[2] + addZ )

viz.callback(viz.MOUSEWHEEL_EVENT,onMouseWheel)