MCNPX's geometric modeling capabilities are limited to Boolean combinations of primitive geometric shapes. These capabilities are not sufficient for simulating particle transport in stellerators, whose geometric models are quite complex. We describe a CAD based implementation of MCNPX, where a CAD geometry engine is used directly for solid model representation and evaluation. The application of this code, to calculating the neutron wall loading distribution () in the Z and toroidal directions for the ARIES-CS design, is described.