Summary: How to add a custom popup menu to Rhino's main menu from a Rhino plug-in.
The CRhinoPlugIn class provides a number of methods that make adding custom popup menus to Rhino easy.
See rhinoSdkPlugIn.h for details on the CRhinoPlugIn class.
Rhino 3 Sample - http://en.wiki.mcneel.com/content/upload/files/MenuTest.zip
Rhino 4 Sample - http://en.wiki.mcneel.com/content/upload/files/MenuTestV4.zip
Creating and handling menus in .NET is slightly different than the C++ SDK. The .NET solution is to create an instance of a RMA.UI.MRhinoPlugInMenu class, populate it with strings and use event handlers to handle when the menu items are clicked or need updating.
VB.NET Sample - http://wiki.mcneel.com/_media/legacy/en/VBMenuTest.zip