my default.aspx.vb is unable to see another myclass.cs file in same folder !
You cannot mix C# and VB.NET code files in the same project. You need to add a new C# class library project containing myclass.cs and then add a reference to this project from your VB project.
EDIT: Apparently I was mistaken. You CAN actually mix VB and C# in a web project if you put the .cs files in the App_Code folder. You learn something new every day :-)
Ensure you declared your class public.
Put your C# class into the App_Code folder.
Copy the .cs file to App_Code folder and you should be able to import it. You can read more about App_Code folder here.