There's probably a more sophisticated way of doing it
but this is what I do (I use Windows Vista, so specific commands are based on that):
1. Install Python of course. Only needs to be done once, then you can forget about it, except for updating as necessary.
2. Put mobiunpack.py and the MOBI file you want to unpack into the same directory.
3. Click on Start - All Programs - Accessories - Command Prompt to open a DOS window.
4. In the DOS window, use the CD (change directory) command to get to the directory where you placed the MOBI file and mobiunpack.py. "CD .." takes you up one level in the directory structure. "CD directory_name" will move you down into the named directory.
5. Type the following command:
That will unpack the HTML and a recreated OPF file from the MOBI file, and place those in a subdirectory with the same name as the MOBI file. If you want to specify a subdirectory name for the resulting files to go in that is different to the MOBI file name, you can do so by adding the desired directory name to the command, like so:
mobiunpack.py mobi_file_name.mobi output_directory_name
I haven't needed to do anything else to use mobiunpack successfully - hopefully it'll prove as straightforward for you too, soparch.