Well I figured it out.
Our story begins with, when you sign up for dashCommerce web hosting the site comes preloaded with the dashCommerce application. Even though I am using my own slightly modified version of DC I left the files there and uploaded my version to a sub domain dev. It appears that even though I was running in the dev sub domain, when you went to checkout it was trying to load checkout.aspx in root folder. When I renamed checkout.aspx in the root folder the error changed to 404 resource not found.
Thus I changed the cart::btnCheckout_Click method to:
Response.Redirect( ResolveUrl("~/checkout.aspx") );
While I was working on this I had the SSL certificate installed. But I still could not get to the checkout page and still got the website certificate warning from IE7 and even after clicking proceed to website anyway, it did not work. I tried to navigate directly to the checkout page and could not get there. I since found out that SSL certificates do not work with sub domains unless you purchase a wildcard certificate for $200.
So I decided to get away from using a sub domain for the test website and decided instead on taking the temporary, information display only page I had built previously and add it to the project under the name tdefault.aspx. I made this and the login page the only pages accessible to anonymous users. Then in the Plesk control panel I changed the first default page to tdefaut.apsx. Thus if someone types in the domain only they get the temporary page and my beta testers can enter a full URL and get to the login page of the test website. I did not want to change the name of the default.aspx page in DC, I figured that would be asking for trouble.
Now everything is working except one little wrinkle, sometimes, not always, when you try to go to the DC default.aspx page you get a 404 resource not found error. I don’t think I will spend much time debugging this issue. I assume it will go away when the site goes live and I remove the tdefault page.
Finally I must include a plug for the dashCommerce hosting service, their customer service has been very responsive.