Which version of MapWinGIS are you using? AddPoint was added as a shorthand for velsto his snippet in v4.9.0.
Here's the documentation: http://www.mapwindow.org/documentation/mapwingis4.9/class_shape.html#a0f5afe1392c783538749617b8b5dd7db
And don't forget you need to add the new polygon to the shapefile before you can see it, using EditAddShape: http://www.mapwindow.org/documentation/mapwingis4.9/group__shapefile__editing.html#ga28c5f35af761b4a264af3bcd01e9befb
Paul
Here's the documentation: http://www.mapwindow.org/documentation/mapwingis4.9/class_shape.html#a0f5afe1392c783538749617b8b5dd7db
And don't forget you need to add the new polygon to the shapefile before you can see it, using EditAddShape: http://www.mapwindow.org/documentation/mapwingis4.9/group__shapefile__editing.html#ga28c5f35af761b4a264af3bcd01e9befb
Paul