If you want to read a file from your project you can for example do it like this:
string fileContent;
StorageFile file = await StorageFile.GetFileFromApplicationUriAsync(new Uri(@"ms-appx:///example.txt"));
using (StreamReader sRead = new StreamReader(await file.OpenStreamForReadAsync()))
fileContent = await sRead.ReadToEndAsync();
Also please ensure that you have set the Build Action of your file as Content (it should be as default).
More about URI schemes you will find here at MSDN.