viewers are free , but you still go throught the normal process.
There is no difference between the viewer or the full blown product. the downloaded installer is the same. it is the licence key that determines what you can and can not do.
So : get a key first (there is nothing physical there, not even a download. it is just a number assigned to your 'account'), then log in to the altium website with your account information, download the installer , deploy.
From inside altium it should pick up your key automatically now. The altium account holds a list of all the keys you own. and you can switch between them on the fly.