Cannot add System.Data.Spatial to my Domain classes

Question

I have an application that use EF5. I have a library project with my domian classes.
I want to have use DbGeaography in one of my classes but I cannot add the System.Data.Spatial reference to my project. It does not appear on the Assemblies. I checked that my library is on .Net 4.5 and it is using that Framework.

Thank you