In iOS 9.2 Apple updated how Enterprise distribution works. If you are running iOS 9.0 or iOS 9.1 then see this historical page instead.
However, since most users would have updated to iOS 9.2.x, the new steps involved are outlined below
1) Install the Enterprise App. At this point the app cannot be run until you change a setting.
2) Go into the Settings app and select in General, then select Device Management
3) Go into the Menu with the name of the enterprise developer and select “Trust”
That is it. Now that you have selected “Trust” you can now run the app on your phone.