@serrious,
Just my bit of respectful clarification. Hopefully it won't muddy the waters.
Most of the folks that have been here a while will talk about the cart, shop, site, etc and mean the includes directory that is immediately under the "root of the cart". In other words if your site it myFabulousProducts.com and everything is in that folder, your includes (cart, shop, catalog) directory would be myFabulousProducts.com/includes. If you installed Zen Cart into the shop directory of your site, the link would be myFabulousProducts.com/shop/includes.
Zen Cart suggests (and requires in 1.5.0) that the admin directory that comes with the download be renamed. If you renamed it to myfabpro, then the "admin" referred to in the forum would be either myFabulousProducts.com/myfabpro or myFabulousProducts.com/shop/myfabpro.
What Conor is pointing out is that there is an includes folder in the site and an includes folder in the "admin". Sometimes the distinction between includes and admin/includes will get folks in trouble.
Hope that helps a little.