The Fortran source code for the interpolation (INTG.FOR) and extraction & conversion (XNTG.FOR) programs must be downloaded and then compiled on your platform to generate compatible executable files (presumably named INTG and XNTG, respectively). The INTG program will interpolate geoid heights from data files using user-provided coordinates. The XNTG program will extract sub-grids from the binary data files, as well as convert back and forth between binary and ASCII, or just give statistics about a data file. To use G99SSS, both the executables and at least one data file must be present. Once you have downloaded the desired data files that cover your region, the XNTG program will be used to convert the ASCII data to binary and the INTG program will recognize which of these files are present.
Currently, these data files are presented in a non-graphical interface only (i.e., user must determine the desired data files based upon the extents of the data files and the region of interest). The interactive graphical interface is under construction and will be completed at a later date.