First, verify that that file is marked a content and that "Copy if newer" is set to true.
Your method should look something like this:
private async void Search(object sender, RoutedEventArgs e)
{
var file = await Windows.ApplicationModel.Package.Current.InstalledLocation.GetFileAsync(@"folder\data.txt");
var stream = await file.OpenStreamForReadAsync();
var rdr = new StreamReader(stream.AsInputStream());
var contents = await rdr.ReadToEndAsync();
}
This will read the contents of your file into a string variable name contents
. From there, you can do whatever you need to with it.
The file access APIs in WindowsRT come from the ".NET for Windows Store apps" profile, so when you're looking on MSDN you need to make sure that the APIs you want to use include that in the list of supported frameworks.