How do I add items to the Finder context menu in Mac OS X?

You may want to take a look at using Services. The Finder appends applicable Services to the end of its contextual menu and, depending on your needs, just fit the bill.

In each Library folder may be a "Contextual Menu Items" folder. Special plugin bundles placed in that folder will be loaded and may place items in contextual menus. Look in Menus.h for the entry points of these plugin bundles.

CMPluginExamineContext
CMPluginHandleSelection
CMPluginPostMenuCleanup

From what I have read, Contextual Menu Items are not supported for 64 bit applications. As of 10.6 most Apple applications, including the Finder, run in 64 bit when available.

There are third party applications that restore contextual menu items on 10.6 if you must have them.