On Mac OS X matlab behaves quite oddly with the track pad zooming in and out very quickly and erratically to remove this behaviour Disable Mouse Scroll Wheel Zoom Behavior.

To disable the zoom behavior for the scroll wheel:

File > 
  Simulink Preferences > 
    Editor Defaults > 
      Scroll wheel controls zooming preference.

If you hold the Ctrl key and use the scroll wheel, the scroll wheel behavior is the opposite of how the preference is set.