Quick instructions to enable S/MIME encryption in Apple Mail on a Mac. (I wrote this for my bro-in-law and thought I would share.)
1. Configure the Apple Mail app to use your preferred email account.
2. Request a certificate for yourself and specify that address. Note that it matters, because it's the only address with which you'll be able to encrypt or decrypt messages. Your email address must match your certificate.http://www.instantssl.com/ssl-certificate-products/free-email-certificate.html
You'll get an email back with a link to "install" it into your Mac. It'll go into your Keychain.
3. Open Apple Mail and send someone message. Check the box in the lower right corner of the address pane. When you hang your mouse over it, it will say "Click to turn on/off digital signing for this message."
4. After that, your conversation will be signed and encrypted. Notice the padlock icon next to the signing button. It will hence become a locked padlock icon.