i have no way of knowing if the database connection details are correct or not, only your host would be able to tell you that.
The testing server specified for this site does not map to the MMHTTPPDB.php URL. Verify that the URL prefix maps to the root of the site.
means that the URL Prefix you entered for the testing server:
does not map to the other settings for the site.
in your initial message, you said the address was:
so that is what you should be using the for the URL Prefix, including the subdirectory.
the URL Prefix needs to be the web address that maps to location on the server that the FTP details are pointing to.
it you where to create a page named test in the root of the dreamweaver site and upload it to the testing server, what would the address be to access that page? that is the URL Prefix