Show menu programmatically in WPF
Get hold of the menu item, and do this :
_menuItem.IsSubmenuOpen = true;
Check out this example on how to open a context menu.
http://www.uxpassion.com/2009/01/how-to-enable-and-show-context-menu-on-left-click-in-wpf/
In summary
You can just call:
YourContextMenu.IsOpen = true;
This will display the context menu, just make sure its associated with a FrameworkElement on which it is displaying)