== Adding the static library’s headers ==
We also need to make sure that our app’s build target can locate the public headers used in this static library. Open the “Build Settings” tab and locate the “User Header Search Paths” setting. Set this to “$(BUILT_PRODUCTS_DIR)” (or “$(BUILT_PRODUCTS_DIR)/static_library_name” if we want to be more specific but then we’ll have to update this setting every time we add another library) and check the “Recursive” check box. Now our built target will search our workspace’s shared build directory to locate linkable header files.
** ALSO SET THE HEADERS TO PUBLIC!!! **
see: http://blog.carbonfive.com/2011/04/04/using-open-source-static-libraries-in-xcode-4/