In fact I’m unity 2 licensed developer. The problem is I have reinstalled Mac OS, and latest Unity, but it thinks the trial is over.
I tried the key from Unity 2 license, but it does not work.
I have sent a mail to support@unity3d.com but got no reply.
Need help… I do not want to pay another $400 before I try Unity 3… 
Unity3D is free…
He may be referring to unity pro trial not working.
Erm. Unity license works only for minor updates. Like i buy now i get to upgrade to 3.4/3.5 for free but when Unity 4 is out, i need to pay an upgrade fee. For your case, you bought a Unity 2 license which if you want to use 3.0 and above you need to pay an upgrade.
And by register and quit? Do you mean when you start Unity. I think you should just click register and type in your email and free will be activated.
Yes but say he wants to trial unity 3.x pro - he can’t for some buggy reason. If he wishes to fix that it is easy enough, just mail support and they will extend the trial for that account (sometimes silly things happen on end user machines).
Thanks for help! It was confusing that I had to first click “Register”, then be redirected to web-page with the field to enter my e-mail and license number, so I did exactly that, but of course it did not work.
Suprise was that I was expected to enter my e-mail and click something like “Request something or bla-bla-bla”.
Stupid me…