I created the following automation addin:

namespace AutomationAddin
{
    [Guid("6652EC43-B48C-428a-A32A-5F2E89B9F305")]

    [ClassInterface(ClassInterfaceType.AutoDual)]
    [ComVisible(true)]
    public class MyFunctions
    {
        public MyFunctions()
        {
        }

        #region UDFs
        public string ToUpperCase(string input)
        {
            return input.ToUpper();
        }
        #endregion

        [ComRegisterFunctionAttribute]
        public static void RegisterFunction(Type type)
        {
            Registry.ClassesRoot.CreateSubKey(
            GetSubKeyName(type, "Programmable"));

            RegistryKey key = Registry.ClassesRoot.OpenSubKey(
              GetSubKeyName(type, "InprocServer32"), true);

            key.SetValue("",
              System.Environment.SystemDirectory + @"\mscoree.dll",
              RegistryValueKind.String);
        }

        [ComUnregisterFunctionAttribute]
        public static void UnregisterFunction(Type type)
        {
            Registry.ClassesRoot.DeleteSubKey(
              GetSubKeyName(type, "Programmable"), false);
        }

        private static string GetSubKeyName(Type type,
          string subKeyName)
        {
            System.Text.StringBuilder s =
              new System.Text.StringBuilder();

            s.Append(@"CLSID\{");
            s.Append(type.GUID.ToString().ToUpper());
            s.Append(@"}\");
            s.Append(subKeyName);

            return s.ToString();
        }  
    }    
}

I build it and it registers just fine. I open excel 2003, go to tools->Add-ins, click on the automation button and the addin appears in the list. I add it and it shows up in the addins list. but, the functions themselves don't appear. If I type it in it doesn't work and if I look in the function wizard, my addin doesn't show up as a category and the functions are not in the list.

I am using excel 2003 on windows 7 x86. I built the project with visual studio 2010. This addin worked fine on windows xp built with visual studio 2008.