If you are looking at setting up S/MIME on OSX so that you can sign emails you send and people can send you encrypted emails, it's not only easy, it's free!
1 . Complete the form here: https://www.instantssl.com/ssl-certificate-products/free-email-certificate.html
2. You'll get an email with more instructions, follow them and download your new (and free!) S/MIME certificate.
3. Open up Keychain Accces, make sure it's unlocked (click the lock in the upper left if it is)
4. With the proper Keychain selected (I created one called S/MIME but, you can just use the login keychain), click on 'File --> Import Items...'
5. Find your downloaded certificate and click the 'Import' button.
6. OSX's Mail.app will automatically load any S/MIME certs when sending out an email from an account with an associated certificate in Keychain Access.
You're done! Enjoy sending verified emails and recieving encrypted email!