CreateInstanceFromAndUnwrap

Hi, anyone know how to make this code work?

Dim obj As MarshalByRefObject =
CType(nd.CreateInstanceFromAndUnwrap("RLCMailer.dll",
"RLCMailer.RLCMailer"), MarshalByRefObject)

or even

Dim obj As Object = nd.CreateInstanceFromAndUnwrap("MyDll.dll",
"MyDll.MyClass")

I get error: MyDll.MyClass IS NOT SERIALIZABLE.

what am I doing wrong. the method "CreateInstance", works, in other
worsds this works:
Dim obj As Object = nd.CreateInstanceFromAndUnwrap("MyDll",
"MyDll.MyClass")

but CreateInstanceFromAndUnwrap (which loads from a file) does not
work for some reasons. is there any reason for that.

thanks.