In the past, when you create your iOS project, Xcode will always hardcode the version number of the SDK in your project.
If you open the project using a newer version of the SDK, you will get the Base SDK Missing error:
To resolve this, simply double-click on the project name (in this example, BasicUI) to open the Info window. In the Info window, select the General tab:
At the bottom of the Info window, select the drop down for Base SDK for All Configurations and select Latest iOS (iOS 4.3). Once this is selected, close the Info window.
Back in Xcode, you should see the SDK number that was selected earlier:
If you don’t see this, close the project and reopen it again. This time, the updated version number should appear.