Actually, it turns out the that the only way to do that was to install a plugin called Finderbar, then use a registry hack to enable it permanently. When I first activated the registry hack, it brought up the explorer tree automatically -- BUT when I restarted, the explorer tree was gone. Turns out the registry tweak was meant to keep the plugin Finderbar permanently enabled and NOT the built-in explorer tree...I'm learning to live with it, it's almost as good.
How do you 'make "explore" the default when double-clicking on my computer'?