Are you just trying to use a standard linkpoint checkout within the cart? I'm not sure about the first data portion of this but I know that Linkpoint is supported and you should be able to craft the checkout with the checkout wizard. Let us know if you have tried to customize the checkout after it has been created.
When you get the page cannot be displayed you should right click on the page and try to view the source to see if it has some xml that it is trying to post back. You may need to test this out in a different browser to see more about the problem.