The default include and library directories will respectively be (for 64-bit Windows):
C:\Program Files (x86)\Simply Fortran\mingw-w64\include
C:\Program Files (x86)\Simply Fortran\mingw-w64\lib
On 32-bit Windows, they won't have the "(x86)" included. The AddOn directories are currently (and erroneously) set to your home directory, but this feature isn't used yet. These paths are dependent on where Simply Fortran was installed, of course.
The actual paths aren't displayed because the paths are constructed automatically when the Makefile is created. Their locations are implied from the compiler location, in the case of the default directories, or the installation procedure, in the case of the AddOn directories. If you'd like to include any third party libraries, I'd suggest keeping them in a folder of your choosing and adding the appropriate directories explicitly to the Search Directories under Project Options.
Does that answer your question?
Jeff Armstrong
Approximatrix, LLC