Did you try the “DRR Image Computation” module from “Radiotherapy” category? You have to create a RtPlan and a RtBeam nodes beforehard in the “External Beam Planning” and “Beams” modules respectively.
The “DRR Image Computation” module has better GUI, shows size and orientation of your imager, and shows plastimatch drr command parameters in the “Plastimatch DRR command arguments” subwindow.