How to? Register offline?

(As usual, I am confused about this, since I have to do it so rarely).
  • Just got a new computer at work.
  • Looked up my activation key for v21.
  • Normal registration can't be done becasue of firewall restrictions.
  • Tried to use the offline method: entered activation key and computer id (this is %COMPUTERNAME%, right?)
  • Offline key was duly generated, entered this into key field on registration dialogue.
  • Still get Connection Refused.
What am I doing wrong?
  • Do I need to unregister on the old computer?
  • If so how? I tried the following on the old machine which has the registred
    • I see Unregister on the dialogue, but it is grayed out, and even after getting around that, nothing happens ...
    • I even entered both activation keys I have (since I can't remember which is which, one is for my personal system) into the dialogue and tried unregister, but nothing happens.

Can you help?