Hi Francisco,
If you have previously installed an earlier version of Animation Paper that had an Activation Key accepted, then you should be fine. It is the same key for you for all Alpha and Beta releases.
If this is your first time installing, then you just need to register to get your Activation Key. Just click the link in the small window coming up when you launch Animation Paper.
Let me know if this helped you or not?
Best,
Niels