1. Visit this website (if the provided method doesn’t work, please visit this website instead).
2. Enter your activation code in the first field
3. Choose your account credentials and click Continue/Create Account.

If you are using multiple activation keys on a single account simply enter the same e-mail address and password. 

NOTE: https://my.nordaccount.com/activate works with 25 digit keys ONLY; https://join.nordsec.com/activate works with 6 digit keys ONLY