Are you getting the error message 'Key is already in use within another app'?
This means that you've purchased the premium license for one DVDVideoSoft application only. You can't activate Free Studio with this key either, as Free Studio is a bundle of all apps.
You need to launch the app you first activated and go to File - Premium to unlock the premium features within it.
Free YouTube Download and Free YouTube to MP3 Converter are 2 different apps
The one-app premium key works on 3 Windows PCs or on 3 Mac. You can't use the same key on different operation systems.
If you want to activate other DVDVideoSoft apps too we recommend you to upgrade your license to All-in-One.
Just launch the confirmation email you received with the Key and accept the offer.
In case you have activated the wrong application, submit a request to our support team. Don't forget to specify your premium code.