I have an IPA signed for ad-hoc distribution. I can install it fine with Flash Builder. I\'d like to install a release version via iTunes 12, though. Dragging it onto:
Note : If you are using iTunes 12.7.0 or above then use Solution 2 else use Solution 1. Solution 1 cannot be used with iTunes 12.7.0 or above since Apps section has been removed from iTunes by Apple
Solution 1 : Using iTunes 12.7 below
Tested on iTunes 12 with Mac OS X (Yosemite) 10.10.3
Also, tested on iTunes 12.3.2.35 with Mac OX X (El Capitan) 10.11.3
This process also applicable for iTunes 12.5.5 with Mac OS X (macOS Sierra) 10.12.3.
You can install IPA file using iTunes 12.x onto device using below steps :





Solution 2 : Using iTunes 12.7 and above
You can use diawi for this purpose.
Drag-and-drop IPA file in empty window. Make sure that last check mark are unselected (recommended due to security concern)
Once the upload is completed then press Send button
link and QR code as well. (You can share this link and QR code with Client)enter this link (Note that link is case-sensitive) OR You can scan the QR using Bakodo iOS appOnce link is loaded you can see app details
Now select ‘Install application’
Press on Install.app installation begins on screen.Edit: See Jayprakash Dubey's answer for iTunes 12.7
From the menu shown in the following screenshot, choose Apps. You can drag and drop you IPA file in the next view.

After that, go to your device's page, you'll see the list of apps, install your app and press Apply from the bottom bar.
For iTunes 12.9.5.5 and above you can install the apps by Copying the IPA file and Paste it (Cmd+V or Edit -> Paste in iTunes) in any categories as Music/Films/TV Programmes etc. The app will be installed automatically on your iPhone screen.
Tested on 29 Nov 2019.
Demo:
I don't remember this being very difficult with iTunes 12, but at least for iTunes 12.8 (and likely for the previous couple of minor versions also) it's pretty straightforward even though the Apps button is not there, as demonstrated in the below two steps:
Wait for the sync to finish and the app is on your device!
IPA file are generally executable file for iOS app. Approach used to install IPA in iTunes 12 is different as compared to lower versions.
Simply, add IPA file into iTunes library and search for IPA in apps section of connected device.
Click on install and Apply.
Tested on iTunes 12.5.3.17
1.Open the iTunes select the “Apps” section with in that select the “Library”
2.Now drag and drop the file AppName.ipa in this Library section (Before connecting your iOS device to your computer machine)
3.Now connect your iOS device to your computer machine, we are able to see our device in iTunes…
4.Select your device go to “Apps” section of your device and search your App in the list of apps with "Install" button infront of it.
5.Now hit the “Install” button and then press the “Done” button in bottom right corner, The “Install” button will turn in to “Will Install” one alert will be shown to you with two options “Don’t Apply”, “Apply”, hit on option “Apply”.
6.The “App installation” will start on your device with progress….
7.Finally the app will be installed on your iOS device and you will be able to use it…