Make sure you added reference to Silverlight version of System.Xml.XPath.dll
. That's required to use HtmlAgilityPack in Windows Phone Project.
I've tested with Windows Phone 7.5 only, but based on this article it seems to be working for Windows Phone 8 project too (see Article Metadata > Compatibility
Platform(s) section in that link). In my case the .dll
resides in this path :
C:\Program Files (x86)\Microsoft SDKs\Silverlight\v4.0\Libraries\Client