If your video is already playing, swipe down on the Touch surface at the top of the remote and select "Subtitles" Choose one of the available options.

You can also adjust Closed Captions from the Settings app at General > Accessibility > Closed Captions & SDH. Click the setting to toggle it on and off.

If you have any additional questions about closed captions, please visit http://media.viacom.com/accessibility.