  We describe an extension to the SOFTSUSY program that provides for the
calculation of the sparticle spectrum in the Next-to-Minimal Supersymmetric
Standard Model (NMSSM), where a chiral superfield that is a singlet of the
Standard Model gauge group is added to the Minimal Supersymmetric Standard
Model (MSSM) fields. Often, a $\mathbb{Z}_{3}$ symmetry is imposed upon the
model. SOFTSUSY can calculate the spectrum in this case as well as the case
where general $\mathbb{Z}_{3}$ violating (denoted as
$\,\mathbf{\backslash}\mkern-11.0mu{\mathbb{Z}}_{3}$) terms are added to the
soft supersymmetry breaking terms and the superpotential. The user provides a
theoretical boundary condition for the couplings and mass terms of the singlet.
Radiative electroweak symmetry breaking data along with electroweak and CKM
matrix data are used as weak-scale boundary conditions. The renormalisation
group equations are solved numerically between the weak scale and a high energy
scale using a nested iterative algorithm. This paper serves as a manual to the
NMSSM mode of the program, detailing the approximations and conventions used.
