<obout:OboutDropDownList runat="server" ID="ddl1" 
     OnSelectedIndexChanged="ddl1_SelectedIndexChanged" AutoPostBack="true" ... />

protected void ddl1_SelectedIndexChanged(object sender, EventArgs e)
{
    label1.Text = "A new country was selected: " + ddl1.SelectedValue;
}

Protected Sub ddl1_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    label1.Text = "A new country was selected: " + ddl1.SelectedValue
End Sub