Interface Builder as a seperate application no longer exists. Instead, its been moved entirely into Xcode.
Click on a .xib file inside of Xcode and it'll show you an interface that's basically the same as Interface Builder was. The working area is in the center, the list of objects is immediately to the left of it, and everything the inspector used to handle is immediately to the right of the working area. The biggest advantage to this system is you no longer have to import / export between .h files and .xib files. Since it's all one application, changing the header to add or remove outlets or actions will instantly be reflected in the interface file.
