Did you use provided solution setup as a template for your code? It already contains a class named Kata, and you need to use it:

public static class Kata
{
  public static string AddBinary(int a, int b)
  {
   // your code ...
  }
}

I tried my code in an editor, and it works just fine, but when I try it here it comes back with:
Fixture.cs(38,29): error CS0103: The name 'Kata' does not exist in the current context
Fixture.cs(44,33): error CS0103: The name 'Kata' does not exist in the current context
Fixture.cs(50,34): error CS0103: The name 'Kata' does not exist in the current context
Fixture.cs(30,49): error CS0103: The name 'Kata' does not exist in the current context

I don't have 'Kata' anywhere in my code...
Any suggestions?