The option to enable or disable "Ignore trackpad when mouse or wireless trackpad is present" on your Mac notebook can be found in System Preferences:
OS X v10.7 Lion or OS X Mountain Lion v10.8
- 

- From the Apple () menu, choose System Preferences. 

- From the View menu, choose Accessibility. 

- Click the Mouse & Trackpad pane.

 

OS X v10.6 Snow Leopard
- 

- From the Apple () menu, choose System Preferences. 

- From the View menu, choose Universal Access. 

- Click the Mouse & Trackpad pane. 

Learn more
To determine if your Mac notebook has a multi-touch trackpad, please see How to determine if your notebook has a Multi-Touch trackpad.
To learn about Macs with multi-touch trackpads and gestures, please see: