I Can't Use [DataMember()] Attribute

[Azthenix]

I can't seem to use the [DataMember()] attribute. When I use it it only shows up as.

The type or namespace name 'DataMemberAttribute' could not be found (are you missing a using directive or an assembly reference?)

[tomsoftware]

[DataMember(Name = "yourNameHere")]is set in using System.Runtime.Serialization;