If you are using Visual Studio 2005 below is how you can enable intellisense to edit your WCF config files. 

Copy the Attached WCF Configuration schema file to your VS installation folder at %Program Files%\Visual Studio 8\Xml\Schemas. You will find DotNetConfig.xsd in the same directory. Open this file in notepad and  following right after the xs:schema element add the below line,

<xs:include schemaLocation="WCFConfig.xsd" /> 

You will now have intellisense support for your WCF configuration files in VS!