Have product key but cant register

 i recently got my laptop from a fair and they provided me with an activation code and told me to just register the code myself.  i just found out that there is no norton internet sercurity preloaded, however , i have tried downloading the trial version but to no avail... i still cant enter my product key . How can i enter the product key?i want to do it asap as i am not free for the next few days.