Are you talking about Hangouts? The Nexus 7 doesn't have an SMS app (at least, the wi-fi only N7 doesn't--can't be sure about the 4G LTE version).
Open the App Drawer and look for Hangouts. If it isn't there, it's possible you might have disabled it. Go to Settings>Apps>Disabled, and select Hangouts if you see it there. Then tap Enable.