Unless you actually created your stuff inside your installation of Django (or under some directory named site-packages or dist-packages), which is not recommended, it is unlikely that the place from which you import Category and Products will be named beginning with "django.".
The book may be assuming more background than you have. If you haven't already done so, doing, one time, the tutorial at
docs.djangoproject.com (be sure to do the one for the version of django that you have installed), is very worthwhile. Things that you may have misinterpreted in the book might then make perfect sense.
And if you're not already facile with python, the tutorial at
docs.python.org, again, be sure to do the one for the version of python that you have, is worth doing first of all.