Well, apparently you didn't understand the problem. The twenty-ten theme has a menu that is named "Primary Navigation" which shows up on the admin interface under "Appearance/Menus" and shows up in a block on that page titled "Theme Locations". Within that block is the following:
Your theme supports 1 menu. Select which menu you would like to use.
along with a dropdown from which you can select the menu for adding menu items. Presumably it's a dropdown so that you can select other menus, should the theme support them.
On my local development site, that "Theme Location" block is available for activating the Primary Navigation menu. Using exactly the same twenty-ten theme with no modules activated on the live site, that block is not available.
For further clarification, I ultimately don't want to use twenty-ten, but a theme in which I've copied that functionality. As I've said, however, I have verified with twenty-ten that there's a problem.
So, hopefully that is an explanation you can understand. Short of totally uninstalling the live site, I don't know how to begin to figure out why there's a problem. Due to the content I've already added, I obviously don't want to go that route.