I think you are misunderstanding something, not sure what.
How to locate the software, activate the shopping cart;
You download the software to your PC using the link on the zen-cart.com homepage, unzip it and upload the complete fileset to your host. On the hosting server, you run the installer by following the directions in the readme and install files packaged with the fileset.
Where to send a user to once the Add Cart button is depressed.
This is part of the Zen Cart system - you don't "send" the user anywhere, Zen Cart does what it needs to do automatically. Or are you thinking you will have an add to cart button on a non-Zen Cart page? That is not the way it works.
The shopping cart page is an integral part of Zen Cart, and there is plenty of talk about various aspects of it.
If you have not yet learned these basic facts about Zen Cart, you need to read about installing in the tutorials at the top of the page. You may be over-thinking things and worrying about processes you haven't gotten to yet; most of these will become obvious when you get there.