I could reproduce this problem in a fresh svn checkout from the website.
When I opened CalendarSample.xcodeproj Xcode said that it couldn't open the GTL.xcodeproj subproject referenced by CalendarSample, so I deleted it from the GTL.xcodeproj project and added it back in:
*) Drag it to the file list.
*) In the CalendarSample project, CalendarSample target, Build Phases panel, Copy Frameworks slat, press the '+' button, and the add the GTL.framework.
*) Drag that Copy Frameworks slat so it is ABOVE the compile slat, so it will be in the right place by the time you get to the compile build phase.
*) Drag the GTL.framework you just added into the linked frameworks slat, so Xcode will know you mean that one.
*) repeat the 'in the Copy Frameworks slat, press the '+' button, and the add the GTL.framework' step, since the previous step removed it.
Now, Xcode knows that this sample should use the private copy of the framework that will be built into this app, both for searching headers at compile time, and for linking at link time, and at load time, it will look for the framework in the Content/Frameworks subfolder of the app bundle.