Newbie here! "no site definition for this document" message DW CS5

I'm trying to get PHP working. I've set up PHP and Apache for Windows on an earlier computer without problem; DW was not involved. I've also set up websites without PHP using Dreamweaver CS3 w/o problems. But I'm having problems with CS5. even though I've carefully followed instructions, tutorials and videos re. setting up a site on Windows so I can program PHP pages. The php editor seems to be working, even though it is flagging errors on some code which I know is good PHP code.

I can't get the site definition right to save my life. I used Wampserver and I found the phpinfo page, so I know that's working.The server is on.  I realize Dreamweaver has to know where the apache server is, and I point it to htdocs, which is buried deeply within the wamps directory. I did not make any changes to the apache or php ini files, since I was not directed to do so in the installation instructions.

I can't navigate, in the browser, using localhost. When I try to set up a php page I get "Dynamically=related files cannot be discovered because there is no site definition for this document".  I've tried removing the folder which would contain my web files and starting a new site definition, but DW brings up the old names as reasons for syncronization problems. Any ideas out there?