If I understood correctly, you want to read .stl files at runtime from a specific folder?
include_directories has nothing to do with where files are searched for at runtime, it only affects where the compiler searches for include files during compile time.
You either need to specify an absolute path to the .stl (something like /source/test.stl), or specify a path relative to the current working directory; the working directory in your case seems to be /source/cmake-build-debug/VTK_importSTL.app/Contents/MacOS/, so a relative path to a file test.stl in /source would be ../../../../test.stl.
Alternatively, you could change the working directory for your application; there are however many ways to run your program (via IDE, via shell, etc…), and this influences how you specify the working directory, so you’d have to tell us how you are actually running the application. I suspect you are running it directly via XCode?
You could set the working folder manually as described here - probably a bit outdated, but it should give an idea where to look.
You should also be able to set the XCode working directory via CMake.