I have an IPA signed for ad-hoc distribution. I can install it fine with Xcode Organizer by dragging it to the device. It also worked with iTunes <=10 by dragging onto th
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
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 +