At the top of the Home page you'll see the Club Iberia Plus tab and the boxes to enter the number of your Club Iberia Plus card number or your email, and just below that your password.
If you don't belong to Club Iberia Plus yet, you can join on Iberia.com by filling in a short registration form.

You can also link your Club Iberia Plus profile to social media (Facebook and LinkedIn) from the same tab.