Are you using the stock Nexus 7 launcher? Or the Google Now Launcher (formally know as GEL or Google Experience Launcher) or a 3rd party Launcher like Nova or Apex??
If your using the stock Launcher, tap the app draw button in your launchers dock (circle with six small circles within). At the top you should see two tabs, apps and widgets. Tap the widget tab, long press on a widget and drop on your homescreen.
If your using the Google Now Launcher then long press the homescreen and select "Widgets".
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