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.
I've been told that John Nack and Adobe are aware of the problem, and I'm pretty sure they're working on it. But moving Photoshop to Cocoa is their #1 task right now. So who knows how long it will take. Or maybe the fix will be in Snow Leopard? We can only hope.