web.config doesn't support "+" or Server.MapPath, try<add key="sqlconnstringProvider" value="Microsoft.Jet.OLEDB.4.0;"/>
<add key="sqlconnstringDataSource" value="/mynss/data/db.mdb"/>then in your code, you dostring sConn = String.Format("Provider={0};Data Source={1};", ConfigurationSettings.AppSettings["sqlconnstringProvider"], Server.MapPath(ConfigurationSettings.AppSettings["sqlconnstringDataSource"]));