Help for installation the 2015 NORTON SECURITY (5 devices) on the third computer!?

Hi, manager,
I bought 2015 NORTON Security software for 5 devices on Feb 16. I installed it on 2 computers. Supposed I have left  3 devices, So I can still it on the another machines. But I can not. Please tell me how to solve this problem. Thanks.  During the installation, the product key is required. I followed the help online, but I don't get the production key. Please help me crying. Thanks.