You need to go to the Cakewalk site and register your product after opening an account and you’ll get online activation through a program called Cakewalk Command Centre. The version you’ve bought is obsolete as Cakewalk went out of business last year. It is still a fully functional though.
I would suggest trying to get my money back from your retailer but having opened the package I doubt you’ll succeed.
The sad part for you is that Cakewalk intellectual property has been bought by a Singapore social media music company called BandLab and just 3 days ago released a version of Sonar equivalent to Sonar Platinum and now called Cakewalk by BandLab. The kicker is it is completely free and in full and ongoing development.