  Recently, we demonstrated the existence of heterotic--string solutions in
which the observable sector effective field theory just below the string scale
reduces to that of the MSSM, with the standard observable gauge group being
just SU(3)_C x SU(2)_L x U(1)_Y and the SU(3)_C x SU(2)_L x U(1)_Y-charged
spectrum of the observable sector consisting solely of the MSSM spectrum.
Associated with this model is a set of distinct flat directions of vacuum
expectation values (VEVs) of non-Abelian singlet fields that all produce solely
the MSSM spectrum. In this paper, we study the effective superpotential induced
by these choices of flat directions. We investigate whether sufficient degrees
of freedom exist in these singlet flat directions to satisfy various
phenomenological constraints imposed by the observed Standard Model data. For
each flat direction, the effective superpotential is given to sixth order. The
variations in the singlet and hidden sector low energy spectrums are analyzed.
We then determine the mass matrices (to all finite orders) for the three
generations of MSSM quarks and leptons. Possible Higgs mu-terms are
investigated. We conclude by considering generalizations of our flat directions
involving VEVs of non-Abelian fields.
