You must use IsolatedStorageSettings.ApplicationSettings.
Save value:
IsolatedStorageSettings appSettings = IsolatedStorageSettings.ApplicationSettings;
appSettings.Add("email", "someone@contoso.com");
appSettings.Save();
Download value:
IsolatedStorageSettings appSettings = IsolatedStorageSettings.ApplicationSettings;
string val = (string)appSettings["email"];
See the MSDN tutorial here: How to save and retrieve application settings using isolated storage . This is the Silverlight desktop, but it works the same on Windows Phone.
EDIT:
IsolatedStorageSettings.ApplicationSettings , ( @RichardSzalay ).
, IsolatedStorageSettings.ApplicationSettings Mutex.
: Windows Phone