Visual studio 无法在App_数据中添加.mdf文件

嗨

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请帮助我，您需要将SQL Server配置为“允许用户实例”

   I am not able to add .mdf file in App_data(vs 2008 web developer).
   If i right click on App_data and try to add new item and select sql server 
   file and click OK.
   I am getting error as

     "Connections to Sql Server files(*.mdf) require SQL Server Express
   2005 to function properly.Please verify the installation of the component
   or download from the url:http://www.microsoft.com/Sqlserver/2005/"

  But i have installed SQL Server Express 2005 still it is giving the same
  error(i have installed sql server after installing vs 2008)