CS0234: The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?)

Linq technology is introduced in .Net 3.0 and above.
If you need use namespace of System.XML.Linq, you need reference the following assembly:
C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\System.Xml.Linq.dll
If you need use namespace of System.Linq, you need to reference this assembly:
C:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll
So, not System.XML.dll you are referring.
And Assembly Reference is not involved with UsingTask task. UsingTask is used to imported a new custom task you developed. It is not related to your issue.

---

Windows Live™: Keep your life in sync. Check it out.