This window should happen:
What was it you did before those plugins were "all gone"?
You should check in two locations:
The second can only be found (in Lion and Mountain Lion) via alt-clicking the Finders' Go menu and selecting 'Library'.
If they are in not in either location, you have (somehow) managed to trash them, and you can do nothing but reinstall, unless they're still sitting in the trash, in which case you can simply drag 'm back to Library/Audio/Plugins/Components.
I don't know if the AU manager does not appear if there are no plugins installed; but I would expect it to (and NOT just show an empty window). The fact that nothing happens I find strange.
You should also try trashing your Logic preferences file (in Users/'you'/Library/Preferences, called com.apple.logic.pro.plist)