If you are on windows, I assume you have downloaded and installed ActiveState's ActivePython 2.X for 32 bit as was recommended in this very thread.
Next, you should know there is a gui version of the program (i.e no command line needed) available called MobiUnpack.pyw which actually runs the mobiunpack.py script for you (which is hidden away in the lib directory beside MobiUnpack.pyw).
To run the gui version, simply double-click on MobiUnpack.pyw
To run the command line version:
Yes, you need to run cmd.exe.
But first thing is to create a new folder to hold the contents of the unpack operation. So create a new folder right beside mobiunpack.py and your .mobi file.
Then run cmd.exe and cd to where mobiunpack.py exists
Then run the following command at the prompt:
python mobiunpack.py YOURMOBIFILEHERE.mobi YOURNEWOUTPUTFOLDER
That should unpack any non-drm Mobi file into its pieces. If you look in YOURNEWOUTPUTFOLDER afterwards you will see the .html images/ .op files.
If you run into trouble simply pm KevinH here and I can try to help.