private void Form1_Load(object sender, EventArgs e)
{
    LoadData();
}
private void btnAdd_Click(object sender, EventArgs e)
{
    new FormAdd().ShowDialog();
    LoadData();
}
private void LoadData()
{
   //load your data
}

private void btnAdd_Click(object sender, EventArgs e)
{
    if (new FormAdd().ShowDialog(this) == System.Windows.Forms.DialogResult.OK)
        LoadData();
}