Orbiting with a three button mouse
The Orbit tool is used heavily when creating and editing models. SketchUp contains a few mouse enhancements and modifier keys to allow easy access to the Orbit tool.
Activating the Orbit tool while in another tool
Click and hold middle mouse button (scroll wheel) on a three-button mouse to temporarily activate the Orbit tool while in any other tool (except the Walk tool).
Note: Click and hold the Control and Command keys simultaneously while clicking and holding the left mouse button to temporarily activate the Orbit tool (if you have only one mouse button) (OS X).
Note: You will also be put in pan temporarily if you click on the middle-mouse wheel and then click and hold the left mouse button or press and hold the Shift key.