Hi Ahsan,
System.Data is not available in Metro Apps (Windows 8 Apps). This is a Microsoft decision. As we are currently only compiling for .NET 3.5, we are currently not in a position to support a portable library format. Regardless, the general advice is that your Windows 8 app shouldn't be talking directly to a database, SQL Server or MongoDB alike. Rather, you should expose access to your data via web services either using REST or Soap.
Let me know if you need further guidance. In 2.0, we'll look into building a Portable library format.