DropDownList1.DataSource = stList;
DropDownList1.DataValueField = "Key";
DropDownList1.DataTextField = "Value";
DropDownList1.DataBind();
[Изменить]
Добавление проверенного рабочего кода:
SortedList<int, string> list = new SortedList<int, string>();
list.Add(1, "Test1");
list.Add(2, "Test2");
dropDownList.DataTextField = "Value";
dropDownList.DataValueField = "Key";
dropDownList.DataSource = list;
dropDownList.DataBind();