If you are absolutely sure the serial key is valid, then enter it again but this time make sure to add it exactly how you've received it. Secondly, configure the Firewall in Windows to allow the application to connect to the Internet. This is mandatory in case it's using online activation.
If none of these work, contact the Support team and ask for an official response regarding your serial. Start the software in demo/trial mode, then go to Help > About and use the contact options to get in touch with them.