I had the same problem after buying McAfee Antivirus for a friend. He thought I bought him a fake package and spent his money on something else. So I had a budden on my shoulders to prove that the copy was legit. I contacted McAfee via a phone call and the response was good. However I will not tell you what they told me because your case may be different. Just go to this page http://service.mcafee.com/faq/country.html and get contacts. I did not buy another copy they quickly fixed the issue.
Also check out other ideas here http://www.techyv.com/questions/mcafee-internet-security-activation-unsuccessful