Konrad, you're trying to use the GUI front-end for mobiunpack. The instructions above were for the command line version (which can be found in a subfolder close by). The command line version will end in .py not .pyw
The reason the GUI is not working for you is because you've installed a minimalist version of Python that apparently does not include the Tkinter module. I always recommend ActiveState's version of Python for Windows users (Python.org recommends ActiveState!). It comes with Tkinter and many other extensions that commonly come preinstalled on most Linux or Mac machines.