Upgrading from:

NumberTheory`SiegelTheta`

SiegelTheta is now available as the newly added built-in Mathematica kernel function SiegelTheta.

This evaluates SiegelTheta:

Version 5.2 << NumberTheory`SiegelTheta`; SiegelTheta[{{1 + I, 2 + I}, {2 + I, -1 + 4 I}}, {1.2, 2.3 + .3 I}]