Regarding Creator, I don't know. You'd better start a new thread in the "Tools Forum":http://developer.qt.nokia.com/forums/viewforum/13/.
Regarding the two libs:
You can link the designer plugin to the shard lib with the widgets. You must add that lib to designer/creator too. I don't know where it has to go, though.
Or create a static version of your widget lib and link that to the plugin. You end up with only the plugin shared lib then, no additional libs to install. That's what I do here.
The place for the shared lib(s) is platform dependent. Please have a look in the docs for that (deployment) resp. refer to the system specifications of your OS.