If you find that you are unable to use your traditional mouse’s wheel to control the camera zoom, be sure to go into the options menu (F11) and disable the “Use scrolling gestures to change direction” option. This will re-enable the mouse wheel’s input.
By default, this option is enabled, turning off mouse-wheel input. This allows players with the Apple Magic Trackpad and the Apple Magic Mouse to use gestures to control camera movement. It also prevents Magic Mouse users from accidentally zooming the camera in or out with each touch of the Magic Mouse.
Currently, there is no in-game-camera or world-map zoom functionality for the Magic Mouse (as mentioned in the known issues). Until we are able to address this, we recommend that players turn off the scrolling gestures option to adjust the zoom, and re-enable the option to lock the zoom level.