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To be able to activate the game you need to use the same serial key and e-mail that were provided during the first installation.

Try filing a ticket with EIC's DRM provider, maybe they will be able help, maybe even change e-mail assigned to the serial key you've purchased:

http://esupport.stardock.com/
 
Hi,
since the game has already been registered to the previous user, you will need the e-mail address that he used when installing the first time. With that e-mail address and the activation key you can install the game.