First ask why you want to enable the demo?

The Admin demo is designed to limit functionality of the Admin and this is only on a limited basis at best ...

This was originally added a long long time ago for people who wanted to demonstrate their Admin but prevent the bulk of destructive features ...

To truely use this, you would want to also go through the code to customize other areas that might be destructive ...

ADMIN_DEMO is a configuration_key in the configuration table ...

Due to the nature of this configuration_key it cannot be accessed easily and you need to use phpMyAdmin to edit the record for the configuration_key of ADMIN_DEMO and change that to 1 ...

Are you sure you are wanting the Admin Demo and not just wanting to add the demo products for testing, reviewing etc. the features of Zen Cart?