In EnSight 10, the scroll wheel behavior in EnSight for Mac OS X was changed to accomodate Mac trackpads, including both the Magic Trackpad used with desktop and the trackpad built in to your macbook pro. With a trackpad now, you can used two fingered scrolling to translate the model up/down and left/right in the plane of the screen. Also added are pinch support to zoom in/out and two fingered rotate to rotate in the plane of the screen. Now with just a macbook, you've got a full six degree of freedom input device.
Unfortunately, that means the scroll wheel behavior has to change to match up, so we choose to favor the new trackpad gestures instead of the old scroll wheel behavior. Another plus is that with a two directional scroll wheel (Apple's Magic Mouse, Mighty Mouse and most third party mice), the scrolling in both directions has meaning. (In the old style, left to right scroll wheel didn't do anything.)
Additionally, if you've got a scroll wheel mouse, it's got a right mouse button, so you can zoom by holding that button and moving the mou0.05