You also need Windows.Forms, if you need it just download it from here.
Yoy will need to install it with gacutil:
sudo gacutil -i System.Windows.Forms.dll
Now you can execute WindowsForm Apps ;)
Lets code..
Open monodevelop and create a new empty project. On the *Solution* window -> references -> Add reference and select System.Windows.Forms.Create a new empty file and rename it to Main.cs and add the next code:
// created on 30/04/2005 at 17:38
using System.Windows.Forms;
class HelloWindowsForms
{
        public static void Main()
        {
                MessageBox.Show ("Hello World!");
        }
}
Well.. after this tipical 'hello world', we will try something more interesting...
// created on 30/04/2005 at 17:38
using System;
using System.Windows.Forms;
class MyForm : System.Windows.Forms.Form
{
 private RadioButton MyRadioButton;
 private System.ComponentModel.Container components = null;
 public MyForm()
 {
  InitializeComponent();
 }
 public static void Main()
 {
  Application.Run(new MyForm());
 }
 private void InitializeComponent()
 {
     this.MyRadioButton = new RadioButton();    
     this.MyRadioButton.Text = "My RadioButton";     
  this.Controls.AddRange(new System.Windows.Forms.Control[]{this.MyRadioButton}); 
 }
}
 
