C#

public class DialogForm : Form
{
  private MainForm parent;
  ...
  public DialogForm(MainForm parent)
  { this.parent = parent;}
  ...  
  void SomeFunction(){
    parent.OleDbConnection.Connect();
  }
}

protected override void OnKeyDown(KeyEventArgs e)
{
  if (e.KeyCode == Keys.Enter) //do your stuff
  else base.OnKeyDown(e);
}

private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
{
	if (e.KeyCode == Keys.Return || e.KeyCode == Keys.Enter)
	{
		// do whatever you want here
		e.Handled = true;
	}
}

private void textBox2_Enter(object sender, System.EventArgs e)
{
	this.AcceptButton = button2;
}

private void textBox2_Leave(object sender, System.EventArgs e)
{
	this.AcceptButton = button1;
}