You can't use FrontPage as a "page builder" the way you can with HTML files, because Zen Cart is PHP and MySQL based. You'd have to install an add-on to FrontPage to get it to show the PHP pages parsed into HTML. But even Microsoft are no longer supporting FrontPage, except with security updates - so why would you want to use it?
Zen Cart pages should be edited in a plain text editor, like PS Pad, Text Pad, Crimson Editor etc. which don't place a lot of non-validating rubbish into the page (which FrontPage does). You should also avoid using Dreamweaver.
If you want to "see" your website working offline (on your PC), then you should turn your PC into an offline web server (Intranet) by installing XAMPP (a 1-click install of an offline web server, with all you need to run a local copy of your online website). It's available from https://www.apachefriends.org/en
Vger