There is a mini view of your Storm located on the top right of your screen
At the bottom of the mini view, you can adjust the zoom slider by clicking and dragging the blue dot
The mini view can be minimized by clicking the small tab at the bottom of the mini view. When the mini view is minimized, the zoom slider will still be visible.
Note: Depending on your device, zooming in and out can also be done using a scroll wheel on a mouse, touch/trackpad gestures, etc.