With adding attributes  like this - Dropdown1.Attributes.Add - you're adding client side event. And if you want to access the selected  item from client script you have to use something like Form1.selID.value.....

But as you say, you want to use SelectedIndexChanged server-side event. So you have to write your client-script in that event  - after it's started. You could add  script with RegisterClientScriptBlock like this:

private void DropDownList1_SelectedIndexChanged(object sender, System.EventArgs e)
{
string strUrlToOpen = DropDownList1.SelectedItem.Value.ToString();
string strOpenWinScript = "<script language='JavaScript'>window.open('" + strUrlToOpen + "','','toolbar=0');</script>";
this.RegisterClientScriptBlock("OpenWinScript",strOpenWinScript);
}