Adding a horizontal separator in a MenuStrip

I'm able to run code like this:

this.menuMain.Items.Add(new ToolStripSeparator());

without any trouble... What kind of error are you getting?

In the space between the two fields you want separated by the divider, type:

-

then hit enter (in the designer)

If you need to do this programmatically you can use the same trick:

contextMenu1.MenuItems.Add(new MenuItem("-"));