Which version of Cubase are you using?
Have you installed the most recent eLCC? http://www.elicenser.net/en/latest_downloads.html
What is your operating system? Windows? Mac? 32-bit? 64-bit? Cubase 32-bit or 64-bit?
You enter the activation code in the eLicenser app.
Slow down, take a deep breath … one step at a time.