You can zoom in or out using either the Toolbar, Mouse Wheel, or Keyboard Hotkeys.

Toolbar:
Zoom to Box - Drag a box
around the area you wish to zoom.
Zoom in Slightly
Zoom to Fit
Zoom Previous
Mouse Wheel:
Scroll the mouse wheel forward to zoom and scroll toward the area that the mouse is hovering over.
Keyboard Hotkeys:
+ Zoom In Continuously
- Zoom Out Continuously
F7 Zoom to Fit
F10 Zoom to Box
F9 Zoom Previous