Make sure that you choose monochrome.ctb plot style in Plot dialog, not others. Color Plot Mode option should be selected:
Also be sure that all colors in the drawing use color name (yellow) or color inded (2). nanoCAD cannot plot in monochrome colors in RGB format (244, 237, 117).
If you are working in nanoCAD 5, please note that this version does not printin monochrome color rasters and objects created on layout.