Hi Photoshop user 18,
You are not telling us if you are getting any error or are you able to launch the software or not. Please answer all the questions asked by kglad above and it will be easier for us to isolate and diagnose the issue.
If it's just the activation you are concerned, the activation screen should pop up when you launch the software and you can enter the serial number to activate it.
Else you can launch the software and navigate to the 'Help' menu and select 'Activate' and enter the serial number when prompted.