System.Windows.Freezable is missing

I have got a Class Library project where I keep some interfaces and classes for my WPF Application and RIA Services.

Into the one of class I want to add a member

Brush BackgroundColor { set; get; }

or

SolidColorBrush BackgroundColor { set; get; }

But I got this error.

Error 2 The type 'System.Windows.Freezable' is defined in an assembly that is not referenced. You must add a reference to assembly 'WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.

What's up?

How to fix it if under Add Reference window I cannot see that assembly at all?