Should have read the documentation in detail where it says:
"The Clone and CopyValuesTo methods are implemented as extension methods in the Extensions class and can be brought into scope by inserting a using directive for the namespace, SimpleMvvmToolkit.ModelExtensions.
Clone uses the DataContractSerializer to serialize the object to and from a memory stream. It will work with entities that do not have the [DataContract] attribute by serializing all public properties."
I had [DataContract] attribute in my model and so I am guessing this was the issue. Removed it and now its working fine.