A thought either call tech support or customer service either can help or uninstall teh currect copy and reinstall from your account again that should work.
Another thought is did you have another antivirus installed previously if so did you remove it via its own removal tool. Failure to do that can mess up activation as well.
Unfortunately, I can't seem to find a contact number to call for customer support. I've clicked on "call" on the McAfee customer support page but it won't provide me with a number. Also, I thought about uninstalling and reinstalling but (stupid question here) it should be able to tell that it's the same computer, right? I mean, I don't want it to take up another one of my registrations.
Customer Service, dealing with all account & subscription questions is here: http://service.mcafee.com/CustomerService.aspx?lc=4105&sg=CS&pt=1
For your area the phone number is (quote):
To speak with a Customer Service representative, call us at 866-622-3911.
Average contact length: 10 minutes
Estimated wait time: 5-10 minutes
Hours of Operation: Available daily, 6am-6pm Pacific
Technical Support, for help with the installation or questions about using the software: http://service.mcafee.com/TechSupportHome.aspx?lc=4105&sg=TS
There is free phone support there too, just follow the prompts. Or you can use Online Chat of course.