"In Logic, I insert a new Audio track, go to Mixer view, click an empty Insert button"
I don't know about the Leopard compatibility, but this won't work in Tiger either. As Rohan points out, Ivory (or any instrument) would require a "software instrument" track, not an audio track. The plugin would have to be instantiated under "I/O," not an empty insert. If you look at the choices available as inserts, AU or otherwise, you'll see all your processing options - eq, reverb, amp modeling, etc., but no instruments.