There servername param is the name of your server that host Reporting Services. If the name of the server hosting your resporting services instance is named MyServer then servername = MyServer.

reportserver is the name of your reporting services report server Virtual Directory. by Default this is reportserver.

pathto is the path to your report. I.E. if your report is in a folder called my folder and the reportname is My report pathto = MyFolder%2fMyReport

So given the example you'd use

="void(window.open('http://MyServer%2freportserver?%2fMyFolder%2fMyReport&rs:Command=Render'))"