The problem is in the Dreamweaver Site Settings.
You should double check the testing server tab of your site settings. Most likely, the URL Prefix is not correct.
Take whatever you entered for the URL Prefix and paste it into the address bar of your browser, you should be able to browse to that address with out getting any browser errors.
See out tutorial for setting a Dreamweaver site for dynamic development on the learning resources page:
We have not updated any of our extensions for Dreamweaver CS5 Compatibility. We are currently working on this and will be releasing updates to each extension as we move through the list.