Yes, you use the interface builder. What are you specifically trying to customize? Your question is incredibly vague. Also, you know you can still download Xcode 4 without mountain lion installed.
Here is a link on how to go about customizing your app’s UI:
EDIT:
You said you wanted to add an image to a Nav bar. Just check the class reference: http://developer.apple.com/library/ios/#documentation/uikit/reference/UINavigationBar_Class/Reference/UINavigationBar.html
Use setBackgroundImage:forBarMetrics:
solved Customizing App Appearance on Xcode 3? [closed]