Just like any other IDE, including a library to the project is done by specifying an #include statement on the main sketch.
Because embedXcode relies on true C++, the library needs also to be mentioned on the main makefile to be compiled.
APP_LIBS_LIST = Wire
USER_LIBS_LIST = myLibrary
Use APP_LIBS_LIST for the application libraries and USER_LIBS_LIST for the user's libraries.
Please refer to Libraries APP_LIBS_LIST and USER_LIBS_LIST Variables.
For more information, please refer to chapter Use the Project • section Add Libraries to the Project of the User Manual.