Go to this link here: Ubisoft.com

Download and Install Ubisoft Connector Client on your PC, wait for installation process to finish.

Start the software. Log in with your existing Account name and Password, if you don't have one just sign in to create new account.

To activate a new retail purchase on Uplay: click on the Settings.

Choose on the Activate Product and follow the instructions on the webpage to complete the process.