How to access XPath builder in Sitecore 8
Developer Center was disappeared in Sitecore 7.5. So in Sitecore 8 you will not able to find it in the start menu. But you still can access the developer center in Sitecore 8 by visiting this URL: http://[SiteUrl]/sitecore/shell/default.aspx?xmlcontrol=IDE
To access the XPath Bulider you can go to this URL:http://[SiteUrl]/sitecore/shell/default.aspx?xmlcontrol=IDE.XPath.Builder