System.XML on XAMARIN forms project

Hi,
I am trying to use XmlDocument class **
and **XmlDocument .Load(..) function
on the Portable Project of XAMARIN.Forms Portable solution with visual studio community.

The compiler says that "The type or namespace name 'XmlDocument' could not be found (are you missing a using directive or an assembly reference?"

If i go in References it don't allow me to add the System.XML namespace (there is no) and if i browse file and go to system.xml.dll it says me that the file could not be added because this component is already automatically referenced by build system.

what i can to do to use the class??

NOTE:
in .Droid and .IOS project there is a referenc to System.xml and in those projects I can use XmlDocument class.