If I'm not mistaken, ZModeler can run on "Primary" screen only. However, you can create a floating viewport (View->Layout->Create floating viewport). You can drag this floating viewport and resize it on your secondary monitor. Yet this is "one time" solution only, as restarting ZModeler (or even reloading your saved .z3d file) will not restore this viewports layout scheme... yet may be opening .z3d could, I don't remember.