I am in the process of learning iOS development, and the tutorial I am using on the using tab bars was written for Xcode 3. In the tutorial, you add a third tab to the two that are created by default. It instructs you to use the attributes page for the tab bar controller to add another item:
Unfortunately, this option is gone in Xcode 4, and the method of adding a new tab is completely different. Here is a step by step demonstration of how to add a Tab Bar Item in Xcode 4.
Hopefully this will help someone who is confused by the change.