How can I add a System.Core reference?

Hi... I'm trying to use a Linq Expression but when I compile, an error show me the following text:

The type System.Linq.Expressions.Expression1<System.Func<T,TProperty>>' is defined in an assembly that is not referenced. Consider adding a reference to assembly `System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' (CS0012)

How can I add a System.Core reference if I'm using a Mac in a iPhone project?

The current System.Core reference shows me a 2.0.5.0 version on xamarin.ios package.