I’ll get straight to the point. This is how you do this for Mac OS X 10.5.6:
1. Download the GNU Privacy Guard installer and the GPG Keychain Access installer: Mac GNU Privacy Guard
2. Install GNU Privacy Guard and the GPG Keychain Access applications
3. Get the latest Sen:te GPG Mail plugin: GPGMail for Apple Mail
4. Read the instructions on this website
Disclaimer: This was put up for my own reference so don’t cry if the instructions don’t make sense or you can’t figure this out. This process stinks but I had to do it this way. It was way less painful than dealing with the PGP Corporation’s program “PGP Desktop”. That program forced my hand into going the open-source way.