From the applicable webpage, click on 'Sign Up', then enter your First and Last Name and Your Email address, then click Continue.
http://www.clio.com for North American users
http://www.clio.eu for European users
Then enter the Promotional and/or Referral Codes and click 'Sign Up'.
If you have a promo code to add after signing up, please contact us at support@clio.com and we are happy to apply this for you.