The settings for this are under the iTunes settings, and those depend on which version of iOS you are using. For 8.1.1:
1. Click on the Settings Button.
2. Scroll down to Notifications.
3. Scroll down to Hero Academy and click on it.
4. Toggle Allow Notifications.
If you are still having trouble, please create a ticket and we will help you track down the issue.
*There is currently a known issue with some verisons of iOS not having push notifications appear in the settings menu. We plan to fix this in an upcoming patch.