After selecting Add File(s)... a standard file dialog will open. Take one of the following actions:
By default, the file dialog will start in the directory where you've saved your project. It is often easiest to save your source code in this same directory.
Note that Fortran files must end in an expected extension to work with Simply Fortran. Fixed-format Fortran or FORTRAN 77 files should end with the .f or .for extension, and free-format Fortran should end in .f90, .f95, .f03, or .f08.
Click Ok, and the file should now be listed under your project folder.