In case of a re-activation, you need to use the same email address you used when activating the game for the first time.If you cannot remember the email you used, please contact our support.
In case of first-time activation, your internet security programs are probably blocking the on-line licensing process. Please disable your firewall, anti-virus and anti-spyware programs. Also, make sure you are running the PMC setup with administrator rights and that you have the UAC (User Account Control) turned off in Windows Vista/7/8.
You can also activate your CD key on Steam. There is no activation limit for Steam version of the DLC.