If you want to reach certain categories of apps with in-app display ads, or if you know the apps you want to target, you should:
- A) Go to “Campaign Exclusions” from the Display Network tab
- B) Create a Display Network campaign targeted to mobile apps
- C) Enable Active View reporting
- D) Create a placement inclusion for mobile apps
Explanation: If you want to target particular categories, expand the app stores under “Mobile app categories” by clicking + , then choose the categories you want by clicking » next to each one. These categories match the category names in each store.
Read more here: https://support.google.com/adwords/answer/1722057?hl=en