What happened exactly (what error message) when you double-clicked on MobiUnpack.pyw? If you have properly installed Python on your machine, double-clicking on the MobiUnpack.pyw file should start up a gui window. To test if that would be the case, right click on the MobiUnpack.pyw file and make sure it is properly associated with the pythonw.exe executable that is part of Python 2.X. You probably have not added the Python executable path to your System PATH environment variable or you are using Python 2.7 from python.org and not the ActiveState version that was recommended.
As for your command line errors, if your file exists, then your issue is one of paths. The mobiunpack.py program looked right beside itself to find the fra-eng.mobi book and could not find it (that is why there is an error message).
To make things a little easier triple check that you have copied mobiunpack.py and your fra-eng.mobi to the exact same folder and that inside that folder (right beside your fra-eng.mobi book and mobiunpack.py), you create a new empty folder called "book".
Then run cmd.exe and cd to the folder where mobiunpack.py, your "book" folder and fra-eng.mobi are.
The type the following command:
python.exe mobiunpack.py fra-eng.mobi book
Once complete, you will find the pieces inside the "book" folder.
If not, then there is probably something wrong with how you have installed Python.