Undefined symbols means that the linker cannot find that function/external variable when it links/binds the application together.
NSApplication is the main class that all application is built upon.
I got the same message when I "deployed" my application for a couple of Quicktime functions and that was because I didn't include the Quicktime.frameworks (that shouldn't be the case for you).
I haven't tried the applescript approach in Xcode but the error message should be the same whichever language you use.
Try to clean all targets before you build, maybe that can solve your problem.