If you get this error when trying to save something in the Django admin, it's probably because you forgot to synchronize the database after adding the admin application. If you are using a pre-1.2 version of Django, you can simply:

$ ./managy.py syncdb

Django 1.2 (which is currently in alpha) adds multi-database support. If you want to synchronize a specific database, you can specify that on the command line:

$ ./managy.py syncdb --database=default