Subscription NIS 2010 for my 3rd computer

Hello,

I need to help. I bought NIS 2010 (version for three computers) 29.12.2009 and install it for my two computers without problems. I planned to use the third installation later when I get third computer. Now I have my third computer, installed NIS 2010 to it but I have problems with Subscription, in doesn't work! My Norton account write me that I installed ALL THREE PRODUCTS but it works two computers only! So I have now the new computer with subscription status 15 days. All my trying to insert the key in "subscription window" like recommended me support are brake because I don't understand where it insert? The window not give me this opportunity.

Please help me what I need to do that NIS works for my 3rd computer?

 Thank you