Where do I pay for / activate Parallels 11?

the last version of Parallels that will run on my system is P11. I've downloaded it from the archive page of past versions.

Unbelieveably the archive downloads page misses the most important piece of information - after you've downloaded it, where do you pay for it and get an activation key? I mean, what's the point of being able to download something then have it sitting useless in your downloads folder?