List all with permalink – not working

Hi there

I’ve done this:

First go to your admin area and click settings, then select permalinks. Add this as a custom permalink; /%postname%/%post_id% Now click save. You should now be able to go to http://my_site/cars-for-sale and see a list of all your vehicles.

but it doesn’t work….

can anyone assist?

http://wordpress.org/extend/plugins/car-demon/