Install IPA with iTunes 11
In iTunes 11 you can go to the view menu, and "Show Sidebar", this will give you the sidebar, that you can drag 'n drop to.
You'll drag 'n drop to the open area that will be near the bottom of the sidebar (I'm typically doing this with both an IPA and a provisioning profile). After you do that, there will be an apps menu that appears in the sidebar with your app in it. Click on that, and you'll see your application in the main view. You can then drag your application from there to your device. Below, please find a video (it's private, so you'll need the URL) that outlines the steps visually: http://youtube.com/watch?v=0ACq4CRpEJ8&feature=youtu.be
I always use the iPhone configuration utility for this. Allows much more control and is faster - you don't have to sync the whole device.
For OS X Yosemite and above, and Xcode 6+
Open Xcode > Window > Devices
Choose your device. You can see the installed application list and add a new one by hitting +
For iTunes 11:
- open your iTunes "Side Bar" by going to View -> Show Side Bar
- drag the mobileprovision and ipa files to your iTunes "Apps" under LIBRARY
- open you device Apps from DEVICES and click install for the application and wait for iTunes to sync