Create an account at https://my.presonus.com
Click where it says "Register A Product" - It's a big orange button near the top of the page.
Type in the Serial Number found on the unit and click Register.
The registration should occur and it will appear in your account.
If the previous owner has not removed the registration, you may be blocked from registering it until the previous owner clears the registration. If you know the person you bought it from ask them to clear the registration from their account and then try again on your account.
If you didn't know the previous owner (you bought it in a store), contact support by visiting https://support.presonus.com