Hi, when you want to compile your plugin, you need (in you .pro file) to point out where the sources and binaries for the target QtCreator are. This is needed for a successful compilation of your plugin.
(And that's why you need first to compile QtCreator itself. I.e. you don't really need the app, just some other files). Also building in debug mode is not recommended, both QtCreator and your plugin should be compiled in Release mode.
Once you have the .dll for you plugin, you can copy it to any Qt installation (along with the .pluginspec file). When QtCreator starts and loads the plugins, it checks that the plugins are built against the same version, right now it's 3.2.1. This is to make sure the shared data structures are intact across DLL boundaries etc.
Last year I built a plugin for "Wordstar emulation":https://github.com/tungware/GeezerEditQt it works fine on Windows, Mac and Linux, but I have to recompile it (and QtCreator) when there's a new Qt release :-(