Same thing happening to me. I've even reinstalled the whole CS4 Design Premium to no avail. I'm told John Nack has been told about it (it crashes when installing Photoshop Extensions or for that matter no matter what I try to do). This means my SlideShow Pro for Flash doesn't work, and I can't fix it.
Adobe really needs to address this issue. There's no excuse for not making it easier to replace a broken Extension Manager installation and start over. It won't do that when you try with the Extension Manager installers.
