For Leopard (10.5.x) or Snow Leopard (10.6.x)
- Click the Apple icon of top left hand corner, select System Preferences
- Click on Network, select Airport and click on Advance
- Make sure you are on Airport tab and get the following screen
For Tiger (10.4.x)
- Click the Apple icon of top left hand corner, select System Preferences
- Click on Network, select Airport under Show to get the following screen
