1.Your problem is generated by the fact that you are trying to use
as parameter and also as return type into your Web Methods. By default only the system primitive types and arrays of these primitive types (like int, float, double, string) can be used.
2.If you want to use your own classes as parameters you have to define them as complex types by using XML definition. Here is a MSDN link about this.
3.I strongly advice you, if is posible, not to use Web Service anymore, but to use WCF in place of them (Windows Communication Foundation), because WCF is a new technology and is more configurable and easy to use for all type of communications between applications.