In order to use System.Runtime.Serialization.Formatters.Soap library you have to add reference to System.Runtime.Serialization.Formatters.Soap.dll, which is a part of
MSMicrosoft.Net\Framework\v1.1 or v2.0
// Note: When building this code, you must reference the
// System.Runtime.Serialization.Formatters.Soap.dll assembly.
using System.Runtime.Serialization.Formatters.Soap;
Be aware that Soap Serializer does not support serializing Generic Types. Do not forget to mark them [NonSerialized].
MSDN