Simply log in to your account. Go to "My Policies," then click the link to your current policy number. Scroll down and you should see a button that says "Add Additional Insured." Click that button, enter required information, click submit, and then you can download and/or print your certificate.
For a detailed walkthrough of how to add an additional insured, log in to your account and go to "Members-only Section" then "How-to Videos."