Read/import existing Excel file programmatically (cell-by-cell) in Windows Phone 8

I am working on a Windows Phone 8 app to READ/WRITE Excel files. I asked a question here about this and the comment provided and many other links led me to OpenXml.

All of this got me good on how to create an Excel file and how to launch it. But now I am stuck at very basic of these all i.e. How to read an existing Excel file (probably created outside using MS Excel) cell-by-cell i.e. I want to access each cells and their values through my code. In the openXML thing I did this:

Stream localFile = App.GetResourceStream(new Uri("/ReadExcel;component/jai.xlsx"
                                                    ,UriKind.Relative)).Stream;
MemoryStream ms = new MemoryStream();
localFile.CopyTo(ms);

DocumentFormat.OpenXml.Packaging.SpreadsheetDocument spreadsheetDoc =
DocumentFormat.OpenXml.Packaging.SpreadsheetDocument.Open(localFile, true);
{
    var a = spreadsheetDoc.Package;
    // Do work here
}

But it gives me error:

The type 'System.IO.Packaging.Package' is defined in an assembly that is not 
referenced. You must add a reference to assembly 'WindowsBase, Version=4.0.0.0

So basically I am stuck at this WindowsBase.dll. I tried all various ways to import an assembly i.e. unblock and all, but nothing works.

So all I want to do is to programmatically access the content of an existing Excel file in my code cell-by-cell.

Please help or suggest whether it is even possible as of now in WP8.