Make sure your "main page" has the wpStoreCart shortcode in it. If you are using version 2.0.5 or above it will prompt you to create these pages and automatically insert the shortcodes in them. If you want to do it manually, goto Pages > Edit Pages then find your main page and past this code in it:
Then do the same for your Checkout page, except use this code:
Again, if you're using wpStoreCart v2.0.5 or higher, you will automatically be prompted to have these two pages created for you.
In the future, please use our wpStoreCart support forums, as we do not actively/regularly monitor this WordPress.org forum at this time. Thanks for checking out our plugin, and feel free to offer feature suggestions at our forum as well.