The "What's New" description on the Play Store page says that the unread badge is for "some Android phones only." I suspect this means that the phones have to have a launcher that includes the unread badge feature (like Samsung's Touchwiz or ASUS's ZenUI, for example). The Google Now Launcher (or older Nexus launcher) doesn't include this feature natively. You could always install a 3rd party launcher that does support unread badges, like Nova Launcher (along with its add-on TeslaUnread).