These are similar questions: How-to: Load a type from a referenced assembly at runtime using a string in Silverlight, GetType on a class in a referenced assembly fails but n
Maybe the referenced assembly isn't loaded at the time. Also, I understand from your question that you do not have the full type name, only the class name.
You should try something along this line then:
Type type = Assembly.Load("YourAssemblyName").GetTypes().First(t => t.Name == "ShortTypeName");
Hope I understood you correctly.