If this is in Program.cs: Visual Basic uses a bit of compiler magic to make Program.vb work. In C#, you'll have to write your own start-up method, the compiler won't generate any for you - you can delete the contents of Program.cs and replace it with a new C# class with a static void Main() that shows the main form (create a new C# project to see how it's done).
If this is somewhere else in your program, you have two possibilities: remove the using statement and rewrite the code that depended on it; or add a reference to Microsoft.VisualBasic.dll to your C# project.