The iOS 9 update no longer gives you an option to trust an untrusted enterprise developer build from within the home screen. Users will receive an “Untrusted Enterprise Developer iPhone Distribution: has not been trusted on this iPhone/iPad. Until this developer has been trusted, their enterprise apps will not be available for use.” popup when opening an enterprise build app. The option to trust is located within the settings for iOS 9+.
How-to Trust “Untrusted Enterprise Developer”
1. Go to Settings > General > Profile
2. Press on the developer’s name
3. Press Trust
4. Press Trust again
You will now be able to run the enterprise build app.