Which iOS version/s does the APE Mobile app support?
The APE Mobile app is normally supported on the most recent 2 versions of iOS.
At the time of writing that is iOS 8 and iOS 9. To check the current App's iOS support, look under "Compatibility" on the APE Mobile App page on the App Store.
If your iPad has an older version of iOS (like 7) then update it to a supported iOS version. The iPad iOS version can be seen under Settings => About, and updated via Settings => Software Update.