You need to have your code inside a class, which you do not have a declaration of. The compiler thinks that using System.Dynamic is your class declaration. For example:
using System.Dynamic;
public class Expander : MonoBehaviour {
dynamic MyDynamic = new System.Dynamic.ExpandoObject();
MyDynamic.A = "A";
}
The class needs to be of the same name of the file that it’s in. This class would need to have a file name of Expander.cs.
I’m not sure what ExpandoObject is, but the error is a compiler error with your code, not with ExpandoObject.