There are three steps to using the certificate:
- Make sure it is installed. Comodo sent you an email with buttons, links and password. I found the button didn't work, and you must use the hyperlink and the password to complete the installation.
- Next, you must export the certificate. Using Firefox, go to Preferences - Advanced - View Certificates. There you will see the certificate from Comodo. Export it.
- In Keychain, import it.
If you've used certificates previously, everything else remains the same in the Mail app.