Make sure you've gone to Settings > Add-ins to install an add-in.
If you don't see the Add-In option in your settings, go to the App Store and make sure your app is up to date. If it is, reboot the app (double-tap the home button, swipe up on Outlook to close it, and then start the app again). This ensures the feature is turned on for your device.
Note: You do NOT need the related app installed. For example, to use Evernote, you do not need to have the Evernote iOS app.