Before windows 8, if application want to non-intrusively inform user that something happens, it displays tray icon, maybe animated. For example, if e-mail application finds a new e-mail, it displays a new tray icon (outlook) or animate it’s own icon in tray (all other e-mail clients).
With new Windows 8 Metro UI it’s a new screen with tiles that supposed to be a main thing user will use to launch and switch apps. Where is no tray area in this mode, but many other things – charms area, something like status area that displays clock and battery usage etc.
According to Microsoft Metro UI design guides – what is the new place for poor e-mail app notification icon?
I think what you want is a notification. There are three kinds of notifications: a tile update, a badge update on the tile or a “toast” in the upper right corner of the screen (when in another application). The various options for these types of notifications are listed here.