I found the answer on the Autodesk discussion groups site.:
To edit the POP0 menu, do the following:
1. CUI to launch the Customize User Interface dialog
2. In the tree view (on the left) expand Shortcut Menus
3. Expand Object Snap Cursor Menu (note POP0 alias is displayed in properties)...
I'm trying to figure out how to edit the contents of the pop0 menu in AutoCAD 2006. This is the default middle button menu if the mbuttonpan variable is set to 0. Anyone had any luck editing it.