Did you use the built in browser to find your testing server folder ?
Under Local info in the site setup you should have this:
The testing server folder should read:
This assumes that you have Windows and IIS installed on drive letter C:
Just below that, your URL prefix should have been filled in automatically to reflect the above details, and should read:
Note the ending \ on the end of the folder path and the / on the end of the testing URL.
I suspect your folder path or URL is missing one of the slashes.