see the documentation, the licensing mechanism has changed a little.
Although our (custom) cloud fully supports 3.x builds as far as the model is concerned, the old mechanism with key.mx files no longer works. You need to follow the instructions on the documentation page; log in to the application, go to the Administration menu and click on "License information". You can use that to request a license key in the support portal, which you will then have to fill in in your app in the same screen.