To answer your question...
Spend more time to make sure that zencart is the right cart for you... there are many many ways to catalogue and sell products on the web.
So long as your webhost supports the requirements to run Zencart, all you have to do is open your site in Frontpage, make a folder for the cart then simply drag and drop the cart files into it. Carry on the Zencart setup as per the normal instructions.
You can link to the cart from your existing pages, but there is no other real integration unless you make it so.
The benefit of using Frontpage is that you can manage and work on the zencart files much faster and easier than when using ftp or similar. The benefit of being able to work on your existing site at the same time is also there of course. Dreamweaver offers similar functionality. Notepad does not.
Rob



