How to easily create an Excel UDF with VSTO Add-in project

What I am trying to do is to create User Defined Functions (UDFs) for Excel using VSTO’s C# “Excel 2007 Add-in”-project type (since I just want to generate some general UDFs). As I am only trying to learn the basics (at this stage anyhow) this is what my code looks like:

using System;
using System.Collections.Generic;
using System.Text;
using System.Xml.Linq;
using Excel = Microsoft.Office.Interop.Excel;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Excel;
using Microsoft.Office.Tools.Excel.Extensions;
using System.Runtime.InteropServices;

namespace ExcelAddIn1
{
    public partial class ThisAddIn
    {
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {}

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {}

        //My UDF
        public static double HeronicCal(int a, int b, int c)
        {
            //first compute S = (a+b+c)/2
            double S = (a + b + c) / 2;    
            double area = Math.Sqrt(S * (S - a) * (S - b) * (S - c));
            return area;
        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }            
        #endregion
    }
}

It compiles fine, and when I run it, Excel pops up with a fresh spreadsheet, and when I look at the “Add-Ins”-list (in Excel options) I can see my add-in on the list (which is set to “Load at Startup). But here comes my problem, when I try to call my UDF from with-in Excel, Excel can’t find the method!

What I would imagine is wrong, is that I have to tag my method as an Excel UDF (using the square bra