First thing to do is to try registering your software in your Account. This will tell you if the serial being entered is correct and for what version of software you have purchased. Try copying and pasting the serial to be sure you have it right.If you are absolutely sure that you are entering the correct serial number and it still gets refused then you will need to contact Customer Support using the link below. They will be able to help you further.
http://www.reallusion.com/CustomerSupport/User/QForm.aspx
Hope you get this problem resolved soon. :)