Did you Buy it Online? Or a Cd?
Please try the below steps before activation ;
Check in control panel – Add/Remove programs and uninstall any other security software that might be installed
Then clear the temporary files and cookies on your Internet explorer.
Run the Mcpreinstall tool from here http://service.mcafee.com/faqdocument.aspx?id=TS100614
Right click on M icon and click on product setup .
Follow the onscreen prompts to activate your subscription