Before using ManyCam in your application (iChat, Photo Booth:) you should run ManyCam. See the ManyCam icon in the menu bar:

In iChat open the "Effects" window (Main menu - Video - Show Video Effects) and Select the ManyCam Webcam Effect.

In the ManyCam Sources window you should see iChat - Quartz plugin. It is the source that is currently used by iChat. For example another source would be the iSight webcam.