I drag the .mobileprovision and app.zip file in the list of applications in itunes, I also get add-cursor, but if release the mouse-button nothing happens. the list is still empty, whats wrong?
Unzip the app.zip. You will get a folder, e.g. MyApp.app. Now drag that folder into iTunes and it should be ok.
As mentioned - unzip it, but don't forget to drag the .mobileprovision too If that doesn't work, check back in your provisioning so it's set to the right device. Common mistake.
The only way I know to do this is to get the .app file in Mac, put it into a folder called "Payload". Then you have to ZIP this folder and change its name from "Payload.zip" to "MyAppName.ipa". Mac will ask if you are sure you want to change the extention, select "YES". Now you can drag and drop this file into itunes.
There's no other way to do it afaik. Windows makes problems with the macosx "fork" files (if you open the .app file there is a folder named "_MACOSX" or similar... and windows breaks this).
You can use this amazing software to generate OTA AD-Hocs!
http://github.com/downloads/pcsaito/iOS-BetaBuilder/BetaBuilderDB.zip/