<connectionStrings>
//方法一
<add name="access_conn" connectionString="Provider=Microsoft.Jet.Oledb.4.0;data source="/>
<add name="access_path" connectionString="~/App_Data/blog.mdb"/>
//方法二
<add name="blogConn" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\\blog.mdb;Persist Security Info=True"/>
</connectionStrings>

string sConn = "";
//调用一
sConn = ConfigurationManager.ConnectionStrings["access_conn"].ConnectionString
  + HttpContext.Current.Server.MapPath(ConfigurationManager.ConnectionStrings["access_path"].ConnectionString);
//调用二
sConn = ConfigurationManager.ConnectionStrings["blogConn"].ConnectionString;
OleDbConnection conn = new OleDbConnection(sConn);
conn.Open();